I need new tires and I don't know what size they are originally suposed to be.

On every vehicle sold in the US, there is a sticker - commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box - but sometimes it is located in the trunk or on the fuel filler door. You can find the information on the Owner's Manual.
If the placard is removed, and you do not have the Manual, you can call the Chevrolet dealer, have the VIN# ready, and they will help you. If you cannot get access to Chevy dealer, then go to Internet @:
http://www.tirerack.com/index_w.jsp
Enter the Vehicle, make and Year. Select tires, and you will find the size of the tire for your car.