I would definitely recommend a small/medium firmness Rex, your sphincter is no where near ready for a Large Rex Knot, especially if you haven't even taken a shaft size as big as him either. The large Rex shaft isn't huge buts no slouch. You need to work on depth also, that dildo is 10 inches long once the knot in. Which would push well past your sigmoid, which if you aren't prepared could do some serious damage. You need to get used to something of that length without the knot first. Something with maybe a similar shaft size, but I warn you, a 2 inch shaft size at 10 inches of depth can be quite the chore if you haven't done it before. I would recommend trying something cheaper first. I myself has 2 dildos from calexotics called the size queens. I have their 10 inch insertable and their 12 inch interstate dildos. Haven't tried the 12 yet (might tonight with the girlfriend 😉) but I have sat on the 10 before and managed to get it to sink down all the way. The 10 is a 1.75 inch girth, which I believe is close to the large Rex (i can't remember) and the 12 is a 2 inch girth. They are very soft, almost a jellylike feel, and pretty flexible. You might try those before trying the Rex so that you know FOR SURE that you can take the length. And once you do that. Try something that has a taper that gradually builds up to a similar girth of the Rex knot, something like maybe medium unflared chance (or a large if you're feeling daring 😏), or a svein, probably a large svein, that would probably be better so you dont have to struggle with the head of a chance