Runner compatibility is the thing that catches replacement orders out, because trays and the frames they slide into are not standardised and a tray from a different system will not sit on the rails. Check the existing unit before buying. Depth then follows the contents, with shallow trays keeping small items findable and deep ones swallowing everything. Standardise on one depth per room. See craft storage, classroom equipment and book boxes.

Plastic tubs and tote trays arranged in a bright classroom storage area with stationery, books and activity supplies.
Plastic Tubs and Tote Trays

Shop plastic tubs and tote trays for classrooms, offices, craft supplies, books, files and storage.