I would say skip the jelly/rubber/TPR plugs and go with silicone--you won't regret it, especially if you plan on using it for a longer period of time. The Bootie
is a favorite around here. The downside to these lesser materials (particularly the jelly and rubber) is that they have to be covered with a condom, and that's just not worth it. The money you save on condoms could be put to better use in buying a higher quality toy in the first place.
Regarding size, I would say start with something a bit smaller and work your way up. 4" in circumference (1.25" in diameter) is a good starting place, it's about the circumference of two fingers, so, if you know you can handle two fingers fine, go for 4" (1.25") or 4.25" (1.35"). If you want to go larger, get something with a gentle taper, such as the Flirt