The pinky mice both mice and rat have little available calcium for a adult garter. Mice pinks have less than rats but as long as the diet includes other available prey it's fine.The garter does look healthy thats why I am surprised to hear of what he feeds her.Even if you go by the 10- 15% feeding guide or the girth of the snake certainly 3 mice pinks is inadequate. So , with that in mind I would think the garter should be larger.