Hi there, first of all you need to make shure what minimum dimensions you`ll need.
The "y" working are length depends on the length of the necks you want to do, bass guitar necks are quite long and a neck through is even longer.
The "x" axis working width depends on the width of the bodies.
The "z" axis height depends on the headstock, angled or flat?
As for the machine itself, there are even some who make guitars with a Carvewright so it all depends on what you want.
Spindle power, the rigidity of the machine itself, motors etc. determine the bit diameter, cutting depth and the speed you`ll be able to cut with.
The faster you want the machine to finish the job, the more it will cost but if you can live with the lack of support and maybe some occasional technical problems, you could look for some routers from china. They usually offer a strong spindle, frame etc. for a fair price and will enable you to use 1/2" bits for faster routing.