There's really no limit to the size. I have doubled on 20s, 22s, and 24s and they all worked well. I have heard that a very small (16") kick might choke, and also that very large kicks (26") may not be as articulate as a 22 or 24. But I have never tried a double on those sizes.
It depends on what type of music you play, as well as the types of figures you wish to execute. If you are more of an "icing on the cake" type player, you might not need to change anything in your tuning. If you really need to articulate some intricate patterns, you may find using a 22 or 20 bass drum more to your liking than 24 or 26. If you are looking for response, tune a little higher.
I fall into the former category. I just tune for optimum "slap" on the batter head, which usually means a somewhat slack head, with just a light tension on the front head. I use a Remo PowerSonic batter head with no padding in the drum.