With your device connected to Desktop Manager, select Backup and Restore, then Advanced. On the right hand pane you will see Device Databases. If you Clear "Calendar - All" it will remove all your calendar entries and you will be able to start from scratch
I got almost 6000 entries cause i got several events that are repeated often (therefore one entry for each repetition). Is that a large number? Does it slow down the phone???
Well I cannot judge since I'm a new BB user and immediately transfered all entries from Outlook to my Bold (Like I did on my Nokia as well as my Pocket PC)