Small inbox is good. It forces one to spend some time thinking and remaining organised.
Endless space is bad. It gets one into the habit of hoarding, never to actually giving a second look to 99.9% of the stuff stored.
But if you don't use something, it's like it was never there, ditto for the space it consumes.
So we actually need much less space for our true needs. We just like to exaggerate what our true needs are.
(More or less same reason dumbphones essentially killed amateur photography. They made taking and storing a photograph far too easy to eventually have any significance).
As someone already mentioned, 1000 is a pretty good size for a forum inbox, as most sites restrict it much more. It can be expanded, but as also already mentioned does anyone actually need to store 1000 messages on a forum as a regular member? I really don't think so. Housekeeping is what's needed to delete all the old PMs you don't need any longer and to get into the habit of deleting messages once read if you don't need to keep them.
Also as someone also said, the sent box does also count towards your limit.