9 volt systems: Use the size of a 9V battery as the "measuring stick":
An NTBT 2 band eq. module is about the same size as a 9V battery. So, for 2 band equalization you should have enough room in the cavity for two 9V batteries (the battery plus the module). These spaces can occur between the pots, next to them or on top of them if that will allow the back plate to close.
The NTMB+ 3 band eq. module is also about the same size as a 9V battery. So, for a 3 band eq. you should have enough room in the cavity for two 9V batteries. Older 3 band eq. systems were split into 2 modules that required more space in the cavity.
The XTCT and TCT 3 band eq. modules are also about the same size as a 9V battery. These systems require an additional module if you want to have mid boost. The mid boost modules (MCT-xxx) are about 1/3 the size of a 9V battery. Note that the TCT family cannot operate on 18V.
The AGB-918, AGDB-918, MPB2-918, AGBP-918, and TC1-6 modules are about 1/3 the size of a 9V battery.
18 volt systems: - add one "battery" to the space requirements of a 9V system.