I don't see any problem with a min username of 1 char. 20 seems like a good max. It allows most first and last names to be used together, or a decent combination of many 2-3 words. If fitting the username into a certain area on the screen is a problem, you might want to chop the max down to 15 or so.
Passwords are another matter. I think min 6 is standard, and I would max at 20 here also. The only real reason to set a max on passwords is so that hackers can't flood your system by submitting multi-megabyte passwords.
[edited by: physics at 5:15 pm (utc) on Jun 9, 2010] [edit reason] edited at users request [/edit]