ONE consideration to make is whether you are making neck-throughs or bolt-ons. If you are making bolt-ons, 24x24 is plenty big enough! Plus, small footprint!
If you are making a guitar with a lower bout larger than 20", I'd like to see it! And even a 24 fret (Two octave) 25.5" scale neck wont be much bigger than 18.5" long.
As far as the Z travel is concerned, what is the length of the bit you intend to use? Any z travel that is greater than the lenth of your bit is superfluous. Probably anything over 2.5" will be plenty enough, but aim for 4"(Allowance for longer bits.)
ANOTHER consideration is time. The more perfect you want the machine to make it, the longer it will take. Sometimes, it's just easier and faster to have the machine rough-cut it, and then you take to it with your spoke-shave and files. I think that's the best of both worlds! Let the machine do what it does best, and leave enough for you to do what only YOU can do best!
No matter what you choose to do, Godspeed!
I would do iut the other way, I would rough cut with a bandsaw then put it on the cnc to get it the way i want it. This way you have screw tabs on the item being cut and not use tape.