Micro - Less than 100ml. Examples: a 35 mm film canister or a tiny storage box typically containing only a logbook or a logsheet. A nano cache is a common sub-type of a micro cache that is less than 10ml and can only hold a small logsheet.
Small- 100ml or larger, but less than 1L. Example: A sandwich-sized plastic container or similar.
Regular- 1L or larger, but less than 20L. Examples: a plastic container or ammo can about the size of a shoebox.