I can't give you exact dimensions but I can tell you the front compartment can hold up to 8 Front Runner boxes. I don't own a Turtleback, yet, but have been in the shop and tested the amount of boxes it should be able to hold. I have seen them install a 63qt ARB fridge on heavy duty slides and it took up 3/4 of the space. So just to give you an idea. There is plenty of storage room unless you try to fill it with a bunch of odd ball shapes and sizes which reduces its efficiency.
You can stack the Front Runner boxes 3 high I believe, W 510mm (20") x L 400mm (15.7") x H 230mm (9").
I am thinking putting that much weight on the back would be a bad idea. What would that do to your tongue weight of the trailer? Check out the Turtlback Trailer site and you can see photos of them being built. There is the front cargo area, the middle is the water tank, some electrical and plumbing and the rear is the galley.