We also have Black Jersey Giants (BJG). My BJG rooster is almost 11 lbs, just turned 1 year old and is expected to grow to 13lbs or larger. As a caution, I would suggest that you not use the wide side of a 2x4 for their roosts. Use a branch or the narrow side of a 2x4 because the weight of a large bird seems to have a higher likelihood of causing a breast blister. Here is my thread on my recent problem with my BJG:
https://www.backyardchickens.com/threads/what-is-this-scabby-enlarged-thing-on-roos-chest.1292202/
The breeder (with decades of experience), also indicated that the breast blister was caused by the wide roost. So, we turned them to the narrow side. In the thread, you'll see many pictures, including the roosts turned to the narrow side with the birds on the roosts, including their feet fully covered by their bodies/feathers.
The wide roosts are not typically going to cause problems for smaller/lighter birds, but the weight size and shape of my BJG was not a good combination with the wide roosts. Keep in mind, the BJG is one of the largest and heaviest breeds of chickens.
According to APA, here are the breeds with standard weights at 10lbs or above for a full grown Cock (Rooster), so only a few breeds get really big:
BJG Cock standard weight: 13 lbs
Brahma Cock standard weight: 12 lbs
Cochin Cock standard weight: 11 lbs
Cornish Cock standard weight: 10.5 lbs
Orpington Cock standard weight: 10 lbs
Nest Boxes: ours are at floor level, and are about 16" wide. Cant remember the height, but over 12". BJG lays fine in these boxes. We also have one kitty litter bucket as a nest box. On its side, so they have the height to stand - but, BJG does not use this nest box, probably too narrow. But several of our birds use this one.
Good Luck, and enjoy your birds!