I'm not very experienced and I've only made one nest box before but I think there needs to be 2-3 inches in each dimension of extra space. You want bunny to be able to get in and turn around but not big enough for her to loung comfortably or use as a litter box.
Hope full more experienced breeders will give you some advice.
Sorry, don't know that actual measurements but you want the doe to be able to jump in, turn around, and nurse the babies with about 2 inches of space on all sides of her.
Our Satin boxes are 18-20" long x 10-11" wide and 10" tall. (Sorry, the kids built them and the size varies a tiny bit.) NowI use them for some of the Brood size Mini Rex.
Can't wait to see photo's of the new bunnies! And I'd recommend replacing bottoms in any used nest box you get. (No telling what germs might be left over!) Give them both a good wash with bleach and water and they will be good as new.