i am married with kids, but i also have a sickly girlfriend on the side that i am now shacked up with. we used to sneak around, but we got caught and I got thrown out. but anyway... my question is: She wants to wear a strap-on and do me in my ass. is there a recommended size we should start with? or does it not really matter? I ask because she came home with a rather intimidating black one and I am worried it will cause anal fissures.