I second Bobert - for guitar at least! I mainly use a heavy brass slide, approx. 2 1/4 inch long, 1/8" sidewall, 3/4 inch bore. I use fairly heavy strings and a high action, and play on a couple of single-cone resonators (both cheapish, a Republic Miniolian wood bodied and an Ozark metal-bodied).

I also play slide banjo! Strings here are much lighter and I use a much lighter slide too - brass again, but a much thinner sidewall - about a millimeter.

I use the slide on my little finger (pinky). It's a loose fit so I hold it steady with my ring finger, which is also used to damp the strings behind the slide. I don't have arthritis (yet) but it I do have family & friends with it so I know where you are coming from. I'd imagine a fairly loose slide would suit better than a tight one? The best bet would be to try a few out at the local music store?

I quite like the sound of glass slides but as Melissa has said above stone floors can be unforgiving if you drop the slide on them...

Good luck with the slide playing,

Pete