When you are composing a PM (or a post in a thread too) the server auto-saves periodically so if you "lose" a message because you tried to preview or send but had reached your stored limit you can simply clear space (there's a facility to backup your PMs as well before you do that) and then re-open "Send New Message" panel. You'll see like in the pic behind the spoiler below and can retrieve whatever has been saved by pressing "Restore Auto-Saved Content", likely that'll be all or at least most of what you'd typed. Sometimes just pressing the 'back' arrow in your browser will get it displayed again as well but it depends on settings and you may have to ok a resubmit.
Spoiler:
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