Not from a structural standpoint. That is all about playability. Lets start with the nut. I have seen 12-string guitars with nuts in the range of 52+ mm (over 2") wide and they have been extremely uncomfortable to play. At the same time a 42 mm (1.65") wide nut will probably be a bit cramped. Lets play around with numbers. Say you have 3 mm between the strings in the same pair. And 3 mm from the last strings to the edge of the fretboard. That leave you with 42 - (6x3) - (2x3) = 18mm to distribute between the string pair and that is 3.6mm! almost the same distance as between the strings in the pair. You can of cause have less space between the strings in the pair, but the range 2.5-3 mm seems to be pretty much standard (check that out, I am by no means an expert on 12-stringers).
My suggestion is that you go down to your local music shop and ask if you can test play some 12-stringers (that is if they have at lest a few in stock) and measure the nuts, including string distance etc, on the most comfy once. And be forthcoming with that you are not going to buy a guitar, you are measuring things for a guitar that you are building. Nothing pisses a shop manager off like someone trying to sneak in and take measurements from something he wants to sell (personal experience, I was young...). But if you are open with what you want, he will probably be interested and maybe you've made a new friend.
If you have the nut size and know what bridge string spread you are going to use, the rest is pure math (or draw it out full scale, or in CAD or whatever)