I've been fucking and being fucked by shemales for years and I've taken them all sizes, as small as Kimber James (I actually DID fuck Kimber in Los Angeles) and as big as Mariana Cordoba, and I've learned a trick or two. Here are some useful tips:

First off, relax. If you're tense or stiff (except for your cock) it's going to hurt.
2. Lay on your side in fetal position, then stretch your bottom leg.
3. Spread your top buttcheek with your hand.
4. Push like you're going to the bathroom, so that your anus stretches.