[Choosing Correct RFI Range] If you are playing 6-handed, how do you decide which RFI range to use? Say, for example, you and three others have 200bb and the remaining two players have 100bb... Do you use a 200bb or the 100bb range?

Although the question above is intentionally simplified, by way of background, I play in a regular 5/10 game that has players with anywhere from 50bbs to 500bbs. Generally, two or three players try to buy close to the big stack as provided by the house rules. My practice has been to select my open sizes to my stack size (say 200bbs) even if there are shorter stacks (50bbs) and larger stacks (500bbs). If I happen to be one of the 500bb players, I will aim for open sizes closer to the average stack.

I realize the situation isn’t always that clear, so any general guidance on how you select your open sizes and ranges based on varying stack sizes would be much appreciated.

I'm rarely in that situation (playing online where most people top up), but I guess I'd play a range according to the average of the various stacks, but when deciding on the average stack, I'd possibly put more weight on the size of the "most active" player (i.e. the guy that's most likely to enter the pot). In many games, this would be the big blind. (When I play tournaments and most players are short, I'm particularly conscious about opening too wide if the SB or BB have "3-bet shoving stacks", whereas I'm less concerned about someone acting straight after me, but that has several players after him (e.g. in middle position) as he's unlikely to jam light when others can wake up with a hand after him).

In general RFI ranges don't change significantly from say 100bb to 200bb.

Like Arty said, you have to place more weight on the player that is most likely to enter the pot. For example if I open the CO I would assume that I'm most likely to play against the BB. However if there's a maniac on the button then I'd start placing more weight on them entering the pot. If there's a 100VPIP fish in the SB then I'd have to assume that they would be in the pot.

If your stack is shorter than everyone in front of you then it doesn't matter at all, since the eff stack is always just your stack.