The other day, I had 100 messages in my U2U, but it still let me receive another message it just brought up a message saying I needed to delete my messages. So basically it let me have more than 100 messages in my U2U. So why can't we just have more U2U space?

No additional space will be considered until after our next software update. More stored u2u's fatten the database and slow performance. Try deleting some of your old messages.

This has already been discussed here:
http://www.powerkiteforum.com/viewthread.php?tid=11485