That larger enclosure looks to have plenty of room to jump/fly down....
...or maybe there's a wall inside that I can't see
here?
Inside pics of that larger enclosure might help us help you.
16" high would be ok for the birds, but it might be a pain for you to clean.
Might also depend on where your nests are, roosts are best located a foot higher than nests so they don't sleep in nests.
My boards are about 30" high and 24" deep, roost is 8" above the board and centered.
The 8" was chosen arbitrarily, IIRC, but I can reach under with my short hoe and the
mesh sifter basket., and it reduces the range for 'overshoots'(poops missing board).
My coop is only 6' wide so I have the ramp, tho many don't use it and just jump down between the board and waterer.
Hope that helps.