Here are pictures of my rack. The largest bin on the front side is slightly over 48" wide so I could store half sheets in there. The biggest advice I’d give is to use large wheels so it’ll be easier to roll when fully loaded. I used 5" wheels and they work, but I’d probably go bigger if doing it again. The interior shelves are a tradeoff between what’s the widest piece that’ll fit in there vs how disorganized and wedged in there pieces get. Boards will most likely get slid in there on end, not flat, so I’d make sure at least one shelf could handle slightly more than 12" wide boards - maybe make the space for the bottom shelf higher than the rest and it’s OK to have one smaller one at the top.
The other thing to know is that you will end up putting too many sheets of plywood on the open side and they will have just enough bend in them that it’ll be easy for them to start falling off. I’ve been tempted to put some kind of strap attached to the top with a weight on it that hangs down over the plywood to help hold it in place.
