I'm considering getting one for my partner, however I want to start off small, seeing as my OH hasn't had anything bigger than the Tracey Cox Beginners Buttplug up there and I don't want to damage anything.
I know from experience with my OH that his prostate isn't too far in after entering, and is there really a huge difference (Except for in size) between the 4.5 and 6.5? Is the 4.5 just a case of "straight to the point" and maybe the 6.5 is for a small amount of penetration too?
In my personal experience I'd prefer something bigger, but then again I'm somewhat of an anal whore if you will haha. I like the feeling of a good stretch and being filled.
I'd say start small, better to find it too small and return it for the larger one, than get the larger one, find it *ouch* too big and have to get the smaller one!