Can I change the back tires a different size?

Yes it will. You can not have more than a 3% variance between the overall diameter of the tires. If you do it will set off the abs light or traction light because it sees a different wheel speed and thinks that the tire is losing traction. 3% is a general rule. It could be more or less with your car. Google a diameter calculator and type in your different tire sizes. It will tell you overall diameter or you can check the specs on a site like tire rack. Alex