8 inches is above average, the average guy's "friend" is about 5 to 7 inches long...anyway, it depens on what you guys do in bed...are you playing rough games, are you having gentle sex, it all depends...
I am a guy and well above average and when I use 'normal' size it can happen that if I'm having a little rougher sex with my gf that it rolls up...but there's an another aspect that can also influence the roll-up which is the girl's body. If she's a little smaller built it is more likely to happen...
it all depends...
Why don't you guys try out several different condoms from different brands, till you have found a condom that doesn't slip off...think about...you better wanna spent some money on condoms that having to spent them on a child...(unless you want one)...