If you don’t know your tire size, you should be able to find it in your owner’s manual and on the side of the tires.
Here’s an example of how to figure it out once you find it:
One possible tire size for a 2010 Pontiac Vibe AWD is P205/55R16. For your purposes, you’ll want to note the two numbers directly following the R. That’s the diameter of your rim in inches. Here, you’d be dealing with 16-inch rims. That means compatible Pontiac Vibe Hubcaps would also need to be 16 inches.