I've got 8 girls and they're large, one is Orpington size. Not as large as your Jersey's but still good sized birds that need a lot of room. My communal nesting box is 4 ft. wide but is about 24 in. tall and about 14 in. deep. It's a bit of an optical illusion as the top is slanted on both the interior and exterior, giving them a tall spot inside so it's easy for them to stand up.
From the inside, you can see how the interior roof portion is slanted and that gives the box a lot of height. The height goes all the way up to the 2 x 4 beam that is immediately under the window.

From the outside, you can see how the exterior portion of the nesting box has a slanted roof that also goes all the way to the beam under the window. That has since been covered in tin. To open the box, the back panel unlatches and swings down.

The nesting boxes were the most difficult aspect of the coop to figure out and we had to keep searching to find pictures that would give us hints on how to construct them. I love how our box turned out and hope that this helps.
From the inside, you can see how the interior roof portion is slanted and that gives the box a lot of height. The height goes all the way up to the 2 x 4 beam that is immediately under the window.
From the outside, you can see how the exterior portion of the nesting box has a slanted roof that also goes all the way to the beam under the window. That has since been covered in tin. To open the box, the back panel unlatches and swings down.
The nesting boxes were the most difficult aspect of the coop to figure out and we had to keep searching to find pictures that would give us hints on how to construct them. I love how our box turned out and hope that this helps.