You should introduce something I have seen on many other forums. A PRO account. I would pay $10 to have more space, lets say 500 or whatever. I am very good at cleaning my box out other than messages I want to keep. Even in my sent items I have things I sent people that I want to keep for reference. Whether it is because I was confirming payment sent, or sending a link to something specific to a person on the forum.
Anyway, I am all about keeping the server fast and usable, but changing the limit without notifying people kinda sucks, what about all the vendors waiting for PM's about products and it takes them a day to realize their box was full. Im pretty sure all the vendors would pay more for a "pro" account so they dont have this issue.
Why punish everyone for the ones that cant manage their own accounts!