I've been coding with PHP for over an year, but I still can't figure out what kind of length would be good to use for the auto_increment fields in my mysql tables. I'm talking about the ID fields here. What are the possible disadvantages of choosing a really huge number for the length? E.g most of the time I pick something like 'MediumInt' with '20' as the length, but i haven't come across any case yet where the ids got even closer to that much.
Sooo.. can you guys give me some fool proof method or formula I can always use when designing my tables?