how to make glasses sizes real?

Hello,

very nice tutorial https://rubygarage.org/blog/arkit-2-tutorial. I have followed it and it's really amazing but I have a question that How can we make the Glasses sizes real? Does it depend on the size of the png image or what? As you know there's a lot of different sizes of the glasses and when I put them in the png they all look the same size on the virtual try. Please advice