I've seen billet aluminum wheels with and without the rubber. I hear that those without rubber tend to get a lot of dings in them from those running studs.
The smaller idler wheels are 5 1/8" in diameter and the larger rears are 7", but if you're going custom, I'd say increase the size a bit more to help reduce slider wear.
Also to consider is the size of bearing and their placement on the stock mounts, in particular between the rails, the inner wheels must be centered between the track lugs. You can read several threads on "slider wear" and "oversized idlers", etc. and you will better understand why.