By deleting that file you'll lose your saved sent emails. Well, you could make a copy of that file and move it somewhere else, but that seems pointless to me. I don't save any sent emails, but I didn't know there was a limit as to how many you could keep.
Anyway, to find the store folder go to.
Tools--->Options-->Maintenance-->Store Folder
That will give you the location of what you're looking for.
Before you do that. Are you sure you still have save a copy of sent emails checked?
Tools-->Options-->Send
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