This is partly 'cause the feedback forum doesn't get enough lovin'.
I have maybe 15 PMs and it says I'm 60% full. Seems to indicate the quote is around 25K, as most PMs are 'round 1K. Not actually an issue for me, since "I rarely if ever get PMs"...
But if all 12000 members of FTR filled a 100K inbox quota, it'd still only be 1.2GB. Surely your hard drive is okay with that?