Phil, I have built one resonator (a tricone and I have CADD/pdf drawings of it) and I own both a spider and biscuit. You kind of build a resonator backwards from a normal acoustic, decide on a cone which will determine your body width (I seriously doubt that you'll fit a 9 inch biscuit or 10 inch spider cone in a 0 or 00 sized guitar). Figure out how you are going to support the cone (ie your cone well). Decide on your scale length and whether you want to compensate it at all (ie. what is your playing style). That will determine the end of the scale and the saddle location. Work back from there, where is the neck to body joint (12th, 14th fret) and from there design your neck.
Based on the playability issues, decide on your fretboard width and radius (most reso's are pretty flat and wide) and the kind of neck joint you want to use (most reso's have a neck stick, I used a bolt on neck on my tricone). Figure out your neck geometry, on most reso's you set the action by working with the neck stick and angle, you have very little adjustment at the bridge). Its a good idea to buy your hardware and make sure its all compatible, then take actual measurements to design cone well, neck angle, etc.
I can help with all of this but you do need to make those decisions that you mentioned. Here is my little family
and the guts of the tricone (a biscuit or spider will have a cone well that is both similar and different, I have pictures)
