If you clear your PM box out on a regular basis then your PM box will never get full anyway.🙂 It only takes a couple of minutes to delete any received PM's after you've dealt with them, and if you untick the box when you send a PM so that you don't keep a copy, then you really shouldn't have a problem with not receiving PM's because your PM box is full.🙂
Anyhow, thank you for the suggestion, we will look into it.🙂