There are two sizes of v9 dust boots... 175mm and 220mm... but what does this mean?

It is basically the measurement from the bit-center to the rear intake port:

Grab a tape-measure and latch the end onto the bit... then pull it back, under the gantry, and find the spot where there is nothing behind the gantry.  

This is the measurement you'll need.  If it's >175mm and <220mm... then go with that size.  If it's <220 then go with that size.

The goal is when the 4" hose goes up into the required Ultimate Hose Clamp 4" attachment... you don't want that hose catching on anything behind your gantry.