Size? Whatever fits. A 3/4" inside diameter fits me well, and I like thick walls for tone, except the Dunlop reproduction Coricidin bottles which make up for it a little by some extra mass in the bottom. I have lots of slides in all kinds of materials, but mostly I stick to what I mention below.

My electric slide style approximates that of Derek Trucks (I say approximates because the man is a giant whose knees I could barely touch), which is to say I use a Coricidin bottle on my third finger. Sometimes I use a ceramic or glass slide the same way. Either standard or open G tuning.

On acoustic I generally use a ceramic or glass side on my third finger unless I'm doing the Leo Kottke thing with a larger slide on my 4th finger to cover all the strings at once in an open tuning. Various open tunings.

Brass is nice for the occasional difference in tone, but it rattles a lot in my hands.

Peace, Mooh.