First off, anal lube. Not regular lube. Makes a huge difference. I’m 6’0, 200lbs and larger than average down there. My wife is 5’1, 105lbs and works out frequently with me so she’s petite and tighter than average. Even if she wasn’t, I advocate for 30-45 minutes of full body foreplay, followed by an orgasm, followed by penetration vaginally for a while and hopefully a second orgasm before anal penetration. He should be doing everything he can to get you ready and relax you. After that, it’s just a matter of working up. Tongue, 1 finger, 2 fingers, small toy and then himself. Small steps to get you worked up to it, and to go slow. Anal shouldn’t be painful at all and if it is, you need to either stop, breathe and relax your muscles before proceeding or go back to foreplay. It takes time to warm up properly but it’s so worth it. My wife and I both enjoy giving and receiving anal, and from my perspective in terms of being the one receiving it, breath work and going slow makes the largest difference. Work up gradually with speed and force, but make sure to communicate every step of the way. I love it because my prostate is there, but my wife loves it because it gives access to her g-spot and a-spot and stimulates her clit, just with anal. It also helps to use a toy for clitoral stimulation during anal penetration.