From my experiences and from some good resources I've read and heard from other anal lovers over the years:
I'd imagine that how deep is too deep depends on the person. I have a set of anal beads that are about ten inches long, and I put them all the way in comfortably and easily without any pain. Some people may not like the sensation, and the important thing with anal play, is to take it slow and pay attention to your partner or what your own body tells you if you're the one being penetrated. If it hurts, stop! With lots of lube and being slow and gentle, you can go pretty deep and be penetrated with things that are relatively large without worrying about tearing anything. Just go slow and gentle and use a lot of lube, and it should be fine. The person being penetrated should relax his or her muscles and not tense up. If you want it and it's being done carefully and responsibly, then there should be no worry. Never use toys not intended for anal use, i.e., toys that don't have a wide enough base to stop the toy at a certain point. There are lots of plugs, dildos and beads available here on Eden that are designed specifically for that.
I don't know of any positions or angles that should be avoided, because again, that's up to what feels good for the person being penetrated. it's often easier to start out on all fours or on your stomach, though. Still, you can be on your back of even on top and still have a great, pleasurable experience. Spooning is also a nice position for anal.
As far as how much is too much? Well, I don't know. Your bum is pretty elasticlike and is designed to stretch and go back. Use your best judgement as you go. I use anal plugs and beads on a regular basis, and I've never had an issue with being too stretched out or it not tightening back up when I'm done.
Some people are "size queens" and love using girthy anal toys. What is right for you personally is a matter of trying things out, and working your way up to larger toys. Nothing should ever be rushed or forced, though. It should feel good, not uncomfortable or painful.
I don't know a lot about prostate toys as a woman who has never used them on myself or a partner, so I'll leave that to someone else who can answer.
Silicone toys are more forgiving than glass and other rigid materials. Before you try out harder, rigid toys, I'd continue with the silicone, and consider these things while you use them.
Are you using a lot of lube? Would more lube get rid of the discomfort? What is the shape of the toy? Would a different shape suit you better? Perhaps you should try a differently shaped toy. How relaxed are you? How turned on are you? If you take things slower and make sure you are totally relaxed, not tensing up, and are really turned on, would that help? What position are you in when you use the toy? Perhaps trying a different position would help.
Not to sound like a broken record, but the key to enjoying anal stimulation are to be relaxed, go slow, use lots and lots of lube, and if a toy or position doesn't work for you, keep trying others until you find the right one. Like anything with sexuality, it's a bit of a journey and trial and error until you find just what you like. The more you discover about your body and how it responds to different stimuli, the easier it will be for you to know which toys will likely suit you and which ones won't. However, you'll probably encounter some duds on the way. Don't let that discourage you though; everyone's anatomy is different, and there is no one golden sex toy that everyone always likes. Even toys that are immensely popular and well received has some people who don't see what the big deal is, and toys that a lot of people don't care for, may just be the one that another person has been looking for to send them to the moon. Keep exploring and have a good time.
Also, just a personal observation, but you seem to be really worried about doing damage to yourself with toys, which might fuel anxiety and make it hard for you to relax with a toy. There's nothing wrong with being concerned for your safety; in fact, it's highly encouraged that you be! There are a lot of reviews on the toys here from our community and a lot of really knowledgeable people writing them. Once you familiarize yourself with Eden and all the great info you can get here, I hope you'll feel more comfortable. There are books and guides too, but I haven't read any myself, so I can't recommend any of them from personal experience, but they might be worth checking out.
I hope this helps!