I didn't know mine was full until meredith pointed it out one time in a thread.
When I went to my box, it said I had reached the limit, which was 200 or 220 (can't remember exactly - it was 2something). I should have taken a print screen image of the message before I deleted the messages... because when I counted the messages, I came up with a bit less than that - not to say I didn't miscount.
I do watch mine, now... naturally, that takes care of the sender's end, but not on the other end when one wants to send a message to a full box.
Thanks, JJ. I'm almost positive I'm not the only one that has run into this problem.