so the main rule with the bath mate is to get the one most similar to what you have now, and upgrade once you've gained enough to do so.
I was average initially, so I started with a hercules, then I upgraded to the X-40. if you get a bigger one than your dimensions work with currently, the girth gains will be happening much more rapidly, and as has been mentioned earlier in the thread, the balls are more likely to be sucked in (which is painful, considering sealed space and unsealed space and the squeeze to get through that)