This is a Gridfinity bin to hold ELEGOO ESP-32 Development Boards, but might fit others.

Dimensions:

7mm “deep”      => So thats 7mm for the board thickness and any connectors on it.
29mm “wide” => For boards that are < 29mm, if they have the headers soldered on then theres a little wiggle room for less if theres no headers then as long as its less wide you are good.