Many of the things you ask about are not truly documented, as the limits are pretty high.
I would not expect to see any normal user hitting limits. Several megs, should work.
For UID, I would not use any "reserved" characgers, such as,
', ",. @, $, %, / etc.
It's good practice to use only lower case for uid, too, though not required. Some parts of vaiious applicaitons may be case sensitive, others not.