It's all about lube, lube, and more lube... then patience, patience and more patience.
Combine the two and even the large ones are gonna be possible Missi. I think the biggest obstacle is the space between your ears. "You're gonna put THAT up my ASS?? NO WAY!!!"
Fortunately there are a wide variety of anal toys out there, in many shapes, sizes and materials. I've even seen granite ones! God, imagine getting THAT lost in your bottom!
The funny thing is that a male bottom is basically no different than a female bottom... so if the girls can do it, you should "rise" to the occassion and meet their challenge. Be a man.. so to speak.