I couldn't quite figure out the right size to set it to & was doing it by trial and error... I had the aspect ratio wrong originally. So if I scale it up, then I have to crop it as well. It was easier to start over because I couldn't figure out how to edit the original values
Maybe I don't quite understand what you're attempting, but if you already created a color matte, can’t changes be made through Effects Controls?
Have you tried the following?
Select the matte on your timeline. In the Effects Controls panel, click the arrow next to Motion to open Scale Height and Scale Width, then uncheck Uniform Scale to make adjustments. It’s not a pixel change, but scaling provided borders behind two separate images in a video I created. I hope that’s what you were looking for, or at least point you in the right direction.