The Max size is surprisingly large but fits nicely at the bottom of a Rubbermaid bin. It fits two full sets of TKL caps nicely. It will also hold a lot of LOTR caps but won't quite completely hold all the Elvish or Black Speech sets - so close but the Black Speech leaves some space bars behind. The Standard size is good too but if you're the sort to get a TKL set and novelties or numpads, you'll soon run into problems. And only the Max size has the depth for bigger caps. Hard acrylic, very transparent, probably a bit brittle but good for display.
The Max size are a great option to store and/or display keycap sets, specially for tall profiles.
As others have mentioned, I also wish there was a lower profile option of the Max size to better fit Cherry profile sets.
Fits a lot of keys, but you have to put some in at 90 degree rotation
This product fits a lot of keys, but MT3 keys don't seem to fit that well in the rows. I had to orient them along the length of the row to get them to actually fit. That's fine, but somewhat annoying because anything bigger than 1u has to be somewhat jammed in there. Since MT3 keycaps are pretty chonky, I am not too worried about it, but I am a little annoyed that they don't actually fit.