You can buy sheets especially for the pack and play, but as far as I understand a lot of generic playard sheets are the same size. You have to check and make sure they aren't for the optional mattress. Sorry I can't help more.
I bought the play yard and sheet together and I'm fairly certain it is a regular play yard size sheet. The one I bought was the jersey material version that was offered as an option as "buy with play yard ". It wraps nicely underneath the mat.
You have to specifically ask for sheets made for playpens and not for cribs. I was able to buy 2 pcs of sheets and they all fit well on my playpen. You may also ask Graco for the size of the playpen and refer to that when buying sheets.