How do I make my Favorites icons fit their squares?
If these are png files you can open them in Preview and adjust the size. Try to keep the proportions the same when you do this. In Preview there is a menu item Tools. Click on the and you'll see Change Size as an option. Click on that and it'll bring up the panel shown below.

On this panel, you can make your image any size you want. I think the icons are square so you can set them as 64x64 for example.