Nothing seems to affect the playability and the way a player feels about the instrument like the neck. It’s the primary contact between you and the guitar. Does it fit comfortably in your hand? Are the strings the correct spacing for your playing style? Are the strings difficult to press or the action seems stiff or high? All of this is extremely important to the player and very carefully measured and constructed by the luthier and tiny changes make a lot of difference.
The good news is like body size, the range of acceptable sizes to most people falls within a fairly narrow spectrum. I’ll list the common range of size and their attributes and how they might contribute or detract from your common playing style.
Necks are available in a number of different shapes; half-round or “C” shaped, "D" shaped, “V” shape or a flattened “C” or “V” shape. If your playing style is one that rests the back of the neck in your palm or the fleshy area between the thumb and forefinger with your thumb often wrapped over the top a “C” , elliptical or flattened “V” shape will probably be most comfortable for you. Bear in mind, that the neck is tapered, growing larger as it approaches the bridge from the nut so a half-round is not a constant radius like holding a pipe, but more like splitting a baseball bat lengthwise with the radius growing larger as it approaches the body. Most people like and are comfortable with a "C" shaped neck.
The “V” shaped neck or modified “V” was popular on some vintage guitars and still preferred by some. Players with small hands might find that it is easier to use the back side of the “V” to anchor their thumb to give them more pressure and leverage for barre chords. A “V” is also better if paired with a more narrow fingerboard. Some feel that the “V” wedges too tightly in the web of the thumb/forefinger and hinders their movement.
A flattened “C”, ellipse or flattened “V” works for some, particularly those who play fingerstyle with their thumb pressing in the center of the neck and their hand dropping below the fingerboard with an arched wrist. Most classical and jazz players tend to play with this hand position.
The average player will most likely be satisfied with a “C” or modified "C". For me personally, within a few minutes of playing, I’m usually not thinking about the neck shape at all. If you have a guitar that you like the neck shape of, it may be wise to choose that shape.
Neck Width and Depth
The neck width is determined by the width of the nut and the center to center spacing of the two outside E strings at the bridge. The strings themselves should be about .050 in from the end of the fret bevels on the fingerboard. Fingerstyle players tend to favor a slightly wider nut than plectrum type players. The differences are very small but very noticeable ranging from 1.7” on a vintage flat picking dreadnought to 1.83” on a 12 string. For a 6 string used as both a fingerstyle and flatpicking instrument, a comfortable midrange is about 1.75”. On the bridge end, the typical spacing from the center to center of the E to E bridge pins ranges is usually 2.125”on a 1.7" nut width and 2.25” for a 6 string guitar with a 1.75" nut width.
Neck depth usually but not always, tapers about .100” from the nut to the heel with the depth about .800” – .860” at the first fret and about .900 "– 1.00” at the 10th fret where the heel starts to blend with the neck.
Unless your playing style or hand size requires it, I’d recommend a “C” shape, a neck width of 1.75” at the nut and 2.25” at the bridge and a neck depth of .800" to .900" at the 1st and 10th fret positions respectively. If you already have a guitar that you like, it can be measured and replicated.
As discussed in the section on wood, fingerboards can be of many different materials as long as they are hard, dense, stable and hold a fret tang well. If you plan to use inlays or markers it is prudent to have a contrasting material so they are clearly visible. Typical fingerboards have a radius or crown on steel string guitars and are flat or nearly flat on classical guitars. The curvature of the board facilitates the use of barre chords.
The tightest radius fingerboard I’m aware of is on early Fender electrics with a radius of 7.25”. The problem with this cylindrical radius while comfortable to play is that if you bend the strings too far, there is a tendency of the string to “fret out” on the frets closer to the bridge from the note being played. In other words, when you push the string over the hill of the fret being played it touches the middle of the fret closer to the bridge. This problem can be overcome by using a flatter fingerboard or by making the fingerboard a section of a cone rather than a section of cylinder. The smallest radius possible without “fretting out” is a cone shape 10” at the nut end and 14” at the bridge.
In practical terms it makes sense to use a compound, conical fingerboard if the radius is less than 14”. If it is 14” or above, it is simpler to use a cylindrical section rather than a conical section. A good middle ground is 12” at the nut and 16” at the bridge and this is what I would recommend.
Much has been written regarding scale length and opinions abound, most of them based on experience with particular instruments whose playability or lack thereof could be influenced by a number of factors other than scale length. From the shortest scale to the longest scale commonly in use, the length is not greater than about 1.2” with the short end being around 24.5” and the upper end being around 25.7”. A short Gibson scale is around 24.56 - 24.75” and a long Martin scale is about 25.4”. In order for there to be a discernible difference to the player, the scale length would need to be altered by about .375”. If you are a moderately accomplished player, you should be able to press down the first fret and drop your pinkie on the 4th fret with the first fret held down and make a clean sound although it is a bit of a stretch. If that is not possible for you, work up the fingerboard playing 2-6, 3-7 etc. until you find a span that does work. On a 25.4” scale moving from 1-4 to 2-5 as your comfort zone would shorten the scale to 25.12” and 3-6 would be 24.86”.
Like most things in life, there are trade offs for selecting scale lengths. Attributes of a shorter scale would include a more comfortable feel for smaller hands, slightly less string tension allowing for a more lightly braced top, shorter push for a bend, stiffer fretting feel due to increased longitudinal stiffness, slightly more treble, slightly less bass and sustain. The converse would be true for a longer scale length. The longer scale length should be more comfortable for bigger hands, have a bit more volume and sustain,require a longer push to bend the note and it generally has a fuller sound.
The somewhat de-facto standard has evolved into a scale length of 25.4” for acoustics which suits most people. Some advocate a slightly longer scale length for fingerstyle or drop D type tunings since the slightly added string tension produces a clear sound and allows detuning to D without making the string sound flabby. 25.6” or 25.7” are typically the upper end of scales that I could recommend.