[Evolution] total amount of memorized mails and maildir

[Tommaso A <mb4n4is8 alice it>]

Hello,
at now I have Evolution 2.28.3 on Ubuntu 10.04 
Recently I had this error message:
ÂImpossibile accodare il messaggio sul file
mbox: /home/tommaso/.evolution/mail/local/Inbox: File troppo
grande(impossible to queue the message on file mbox: ... file too big)Â
and couldn't download any more mails, also if I had a big deal of hard
disk space.
a)
-There is a defined quote in KBites or nr. of messages allowed? 
or 
-there is some trick to go around this limit?
b)
- Yes I can backup, but reloading a backup I also reload old settings;
- there is some way to backup only mails and their enclosures?
c)
About maildir with Evolution
-There is a tutorial or guide to set up an Evolution maildir storage on
a USB pen-drive? 
-and what are its amount limits?

Thanks for the answer
Tommaso Aversa
Roma - Italia