Some thoughts on neck Geometry and playability.
The neck of the bass is like the engine of your car. A properly proportioned fingerboard and neck, can be the difference between driving a high performance sports car and a smoke belching bucket of bolts.
Fingerboard: Whatever the material the fingerboard is made from it needs to fill two basic functions. The first is it needs to be stiff enough that along with the neck it is glued to, it can resist deformation from the long term tension of the strings. When properly made, the fingerboard, despite having smaller dimensions than the neck, usually is stiffer than the neck, and together they form an even stiffer beam. If the fingerboard/neck joint is not solidly glued the chances are excellent that neither of therm acting independently is stiff enough and they will both warp. This can be mitigated by a carbon fiber rod inlaid in the neck, to a certain extent, but you still need a thick enough, workable, hard and stable material to adjust the fingerboard for the best playability of the neck.
The second thing to know about the fingerboard is the ideal shape, both lengthwise and in the cross section curves. In order to cleanly and evenly get a good sound out of your bass the overall lengthwise geometry is vital. We call the long shallow convexity over the length of the fingerboard the “Scoop”. Bowed instruments need a certain amount of scoop in order for them to play correctly. Too much scoop and the string is hard to push down, too little or unevenly made scoop and the notes will rattle, buzz or “die.” In addition, the correct amount of scoop can add to something bassists call “growl” a desirable sound in pizzicato playing, much sought after in good basses. The crosswise curves are designed to match the right hand’s ideal ergonomics when changing the path of the bow in crossing quickly between strings. It’s a complicated subject in and of itself, and I recommend looking here for some sound thinking on why this cross radius gets tighter towards the nut of the fingerboard than the other end. The spacing from string to string also effects string crossings and ease of play, but there are normal parameters that work for almost every player, and it’s getting a bit too in-depth to get into it here. There are some other fine points in making a really comfortable fingerboard. Width and taper are of course very important, but an under appreciated detail that makes a big difference in how playing feels is the way the edge of the fingerboard blends into the profile of the neck. A comfortable, rounded transition adds a lot to comfort.
About String height over the fingerboard and how it interacts with scoop: We normally measure string height as the gap between the bottom of the string and the surface of the fingerboard, measured at the far end of the board (closest to the bridge). Different bassists’ playing styles and different tension strings need different string heights. Here are a few guidelines: What I call “medium low action,” (the one that feels best to the majority of bassists I work with) is normally 5mm for the G, 6mm for the D, 7mm for the A, and 8mm for the E strings. Many bassists use about a millimeter higher than this for each string, and few even prefer even higher string heights, or use lower tension strings, like gut or some modern synthetics that really require up to 3 or 4 millimeters higher. A few bassists may want somewhat lower heights, but unless the technique used for the right hand or bow is very light, the tone and dynamic range of the bass will quickly start to suffer with lower string clearances. Here’s a very important thought experiment to go through while we talk about string height numbers. Imagine a fingerboard with a “scoop” of two inches measured at the center of the board. Even if the string clearance was zero measured at the far end of the board, the strings would be almost impossible to push down to the board half way up the neck. Therefore string height is not significant as a measure of the perceived “action” of the fingerboard independently of scoop! One more IMPORTANT note about the string height: at the pegbox end is the ebony Nut that spaces and regulates the birth of the strings over the fingerboard at that end. It's important to know that nuts can only cause buzzes or false tones on the open, unfingered string (usually form a too low or poorly shaped slot) Also that the first semitone is very hard to finger unless the string notch at the nut is very low, normally the height over the fingerboard of a standard business card.
Okay, I know this is getting a bit geeky, but still there are a couple of other things we should at least touch on. The overall depth of the neck and fingerboard and how the curves of the neck are shaped can not only add to overall playability and comfort, but give the bassist important proprioceptive cues for intonation of the neck. The most important of these cues is the “heel stop” (and to a lesser extent the other end or “pegbox stop”), the place where the neck stops being an even taper and transitions into the heel curve. We classify most bass necks as “D necks” or “Eb necks.” This means, if you slide your thumb up the back of the neck to the “heel stop” what note is directly 90 agrees across the fingerboard on the “G” string? Is it an octave above the open D string note, or is it higher (or lower) Most classically trained bassists expect it to be “D” and the stop should not be a large “mushy” spot, but a definite tight radius the thumb can cleanly stop against, with the D not in tune directly across from the stop. Many other bassists might argue for a higher Eb note there because it allows the second, third and fourth fingers of the left hand to reach higher notes without removing the thumb from its parking spot at the stop and assuming another position. It’s all a matter of opinion and mostly what the bassist is accustomed to.
About "overstand" and other neck set details:: Overstand is the amount of neck that projects above the top vertically at the neck/body juncture. some older instruments have a very small height there, about 10mm or so. Most "standard" basses have an overstand of 25 to 28mm. The higher the overstand within practical limits, the easier it is to play the highest positions on the fingerboard, the "thumb positions" (often played with the thumb acting like a partial capo) because it improves the ergonomics to that part of the fingerboard. Many good bass luthiers now use a height here exceeding 30mm. worth mentioning here but a separate question is neck angle which is independent of the overstand and works in concert with it , along with the saddle height, to regulate the angle the strings pass over the bridge (down pressure on the top! an important tonal consideration) and the bridge height.
One more thing. In my opinion, and the opinion of experts in ergonomics, a too small (or too large) profile for the width and depth of the neck/fingerboard can lead to hand problems over time. American made basses like the Kay, King, and Epiphone had very slim, shallow necks, which feel at first more familiar to guitarists who switch to the double bass, but are not ideal for most people in the long run. A too bulky neck can pretty easily be carved down, it’s a job I frequently do on over bulky Chinese made basses. Adding depth and width requires a new fingerboard, and there are limits to what can be done.