It's not exactly a plug right off the bat, but have you looked at the "Aneros Progasm" prostate massagers? They come in three different sizes/shapes as well (the differences are more subtle), and they are good for solo use because there's a handle that will prevent either slipping in too far, and they don't tend to slip out. They are a great way to acclimate oneself to anal play in general and designed for males.
As for plugs themselves...yes, what the others said, you'll want to start small and work up. Between glass and something that actually gives...I guess that's personal. I need something to give to be comfortable and have never liked glass, but it's easily just personal preference. Maybe even differences in anatomy...I've known men to say they need something really firm on the prostate to get it feeling awesome.
Typically I write in Sci-Fi & Fantasy settings and characters are often non-human. The action runs from consent to non-consent to BDSM to group sex and more.
Variety is the spice of life. My Stories
"Let me tell you the plot of every one of his damned stories. Somebody wanted something. That's the story. Mostly, they get it, too." ---Desire, Endless Nights, Neil Gaiman