I couldn't find a list anywhere explaining this, so here you go. The simplest way to parse these numbers is to remember that the first two digits is the width of the unit (not the diameter of the fan) and the second two digits is the depth of the unit. For example, a 5015 fan is 50mm wide by 15mm deep. Note that there are some non-standard sizes available, like the 6025.
- 3010
- 4020
- 5015 (used a LOT for 3D printer cooling)
- 6028
- 7530
- 9733 (standard size for BBQ blowers)