Car seat cover size

Seat covers are sold by make and model. Any site that sells seat covers will have a look up tool.

Yes I've seen that the site I was looking at says it for it only gives measurements on the front seats, the back seat bottom and the rear back rest. I just want to make sure that the measurements are the same

Then I would suggest to get your tape measure out and measure the seats in your Sonic. There are no lists that have that information that I can point you towards.