If you are allowed 10x10 without a permit (same here!), I would suggest building 10x10 rather than smaller as long as budget permits. You will NEVER regret it, I promise... additions are not nearly as cost-effective nor easy, and there are any number of reasons you might in future want more space.
A square is, as others have said, the most efficient in some ways (primarily in terms of siding). However, 8x12 has an awful lot to recommend it -- it gives you only 4 sq ft less (so really, it is the same size), requires trusses that span only 8' rather than 10' and thus can be a bit more lightly built, and will not require inconvenient fractions of plywood or other sheet goods. It also gives you a longer dimension along which to run a single roost, a difference of about 3 chickens' worth which is not entirely trivial. Personally I would be inclined to build the 8x12 rather than a 10x10, although it is not a *big* difference.
In that general shape/size "food group" of coops, I don't honestly believe there is much of a difference in userfriendliness or efficiency of different designs as long as you avoid doing obviously stupid things like putting the waterer under the roost with no droppings board or shield between 'em
Good luck, have fun,
Pat
A square is, as others have said, the most efficient in some ways (primarily in terms of siding). However, 8x12 has an awful lot to recommend it -- it gives you only 4 sq ft less (so really, it is the same size), requires trusses that span only 8' rather than 10' and thus can be a bit more lightly built, and will not require inconvenient fractions of plywood or other sheet goods. It also gives you a longer dimension along which to run a single roost, a difference of about 3 chickens' worth which is not entirely trivial. Personally I would be inclined to build the 8x12 rather than a 10x10, although it is not a *big* difference.
In that general shape/size "food group" of coops, I don't honestly believe there is much of a difference in userfriendliness or efficiency of different designs as long as you avoid doing obviously stupid things like putting the waterer under the roost with no droppings board or shield between 'em

Good luck, have fun,
Pat