Yes, if you want 5 sq ft apiece for 12 chickens then an 8x8 coop is the most sensible way to go. You would want 60 sq ft from your plans; 8x8 will give you 64 sq ft in theory but 8x8 is usually *exterior* dimensions and you will need to subtract 8" or so, each direction, for the actual interior measurements, if you use standard stud wall construction, although if you leave the inside stud structure open then it may not actually be *much* different from 64 sq ft. There is nothing magic about *exact* numbers and that is certainly ballpark-the-same. Build it according to multiples of standard lumber when at all possible!
Good luck, have fun,
Pat