To my thinking there’s basically two optimal RepRap sizes (in terms of build area):

• A small RepRap with a build area just large enough to replicate it’s largest part, one at a time
• A large RepRap with a build area large enough to print all of it’s own parts in a single print job

Random tangent:  wouldn’t it be cool if all RepRaps came with the designs for their own printed parts already on board?  Just fire it up for the first time, calibrate, and start printing replacement parts.  (I realize this isn’t quite feasible – in order to be ready-printable the files would need to be in S3G format and the machine would have to run the STL’s through it’s own Skeinforge settings).