If your anus "too tight" for even a finger or smaller tools, that's more an issue in your head than your ass. OKCLane has it right, it's about going slowly and being patient because physically your anus should easily pass things out that are comparable in size to what you're trying to put in. This means it's more about psychology and relaxation in the moment.
Just use lots of lube, start with reasonable sized toys -- and make sure they're "safe" toys made of metal or silicone, not those jelly-type plastics which usually shed dangerous chemicals (remember: if it smells strongly like plastic, that's because it's leaching chemicals). Go slowly. When you are trying to use a larger toy, set the expectation for yourself that you won't get it completely in the first 2 or 3 times you try. You're just stretching and testing. Don't force it. A toy should slide in easily; if not, you're going to fast.
Fun fact: people who engage in fisting don't have any trouble maintaining continence, and find that after even just a few months after stopping the practice, they start tightening back up noticeably. (Maybe not returning to original size... but certainly tighter than their max size.) So don't worry that some stretching is going to harm you, you'll never lose control of your bowels from anal play -- the only harm is going too fast.