In general when it comes to anal play, some people have an easier time with girth and some people have an easier time with length. Thus for some strap-on play is the best thing ever, and for some fisting is way better. Just depends on your ass' preference, and everyone's ass is a little different.
As long as you go slow, use steady/even pressure, and use lots and lots of lube, you probably won't hurt someone. Especially if you and they are both sober and focusing on what is happening so that you can communicate about discomforts. It's important to be really clean for fisting because any particulate matter in the ass can cause abrasions on the walls of the rectum since the fist provides a lot of outward pressure on the walls of the rectum. Crisco is honestly the most highly recommended lubricant for anal fisting (your ass absorbs it within a day or two much like with any other type of food, and it doesn't dry up easily like water-based does), but since it degrades latex, it is recommended to change your gloves every ten minutes or so. And I DO recommend wearing gloves (get sexy black ones that are your exact size because wrinkles in your gloves can cause problems), especially if you have any cuts or abrasions whatsoever on your hands. HIV and other blood-born pathogens can easily get into your bloodstream when you are fisting someone if you have cuts on your hands. Try also to keep towels and wet wipes nearby because when you are using such incredible amounts of lube it tends to get everywhere and used lube can contain STI's, etc. I recommend pre-tearing paper towels and laying them in a pile where they are easy to reach, or pre-unfolding wet wipes and laying them in a pile within arm's reach, etc. The keys to healthy anal play when it comes to fisting are: make sure the ass is clean, go slow and steady, use tons of lube, keep excess lubrication contained, wear the correct size gloves, and most importantly, THE ASS IS IN CHARGE! It sets the speed and determines when you can move forward. If you are fisting something in conjunction with a "forced" roleplay or domination scene, remember, a roleplay is a fantasy for a reason, and even if you are pretending to be forceful, it is critical to let the ass run the show as far as how fast things move forward.
Hopefully this is helpful to someone out there.