Did you make sure to download them as a file before you deleted them? You can do that. The dropdown list where you move or delete them can also let you download them as XML, comma-separated values, or plain ol' text.
Click the checkbox at the top of the list (next to Sent Date) to check all messages
Well, 5,577 entries are being stored in the pmtext table and it's 3.3 megs, so I'd guess a little over 60kb for average PM's. There are 100 PM's max per page
It would probably help more if people paid a penny for every PM they received There have been 11,715 PM's exchanged between everyone here since the start of OTC.