Everyone is different but your prostate which if stimulated through your anus will make you cum. The prostate is 3" to 5" inside your rectum on the upper side towards your abdomen. When I first started doing this I used varying sized dildos until I found one that both "filled" me up and "vibrated."
My wife makes me orgasm anally twice a day. It really isn't hard to do once oyu know how. Alot of it is learning to relax and go with it, you kinda have to "let go" of your cum. For me it is alot easier with huge toys, but that's becuz of taking it for 14 yrs. It taes really heavy pressure and pounding for me to get over the top, but it is well worth it, it is a VERY intense orgasm if done right. It is also alot easier if someone else is holding the toy, it lets you just relax and enjoy the feeling and wait for the orgasm.
I've tried for many years to achieve prostate milking and orgasm through anal stimulation, I have even bought specific toys none of which have worked. I'm quite dissapointed really.