I delete old PMs one at a time. That's why my box is chronically nearly full. I have no plans to change that yet. (or probably ever)

I second Doc John's post.

I think unaluna's suggestion might result in ALL the PMs being deleted, not just the older ones, but I could be wrong. I did similar once somewhere else. I lost a whole lot of stuff I had wanted to keep.

I would agree with Divine's post - ''delete selected messages''