Like raving said, It all depends, but I'll answer for at least A specific situation. With butted mail, the biggest you'll want to use with most weaves still looking tight is about 1/4". You could probably go a bit below 3/16", but much more than that would lead to rings that are a bit too small.
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That really depends on what weave you're using (if this question is about ease of weaving and flexibility) and/or whether you're making butted, riveted or welded maille (if this question is about durability under simulated combat situations. That said, here's a very good link that might help you figure out what you need.