A useful free goodie from Dennis Porter.

This utility node for Substance Designer from the artist will let you automatically resizes linked bitmap images to the correct proportions.

You just need to know the image's pixel dimensions - Resize Bitmap will do the rest.

The pack also features SBSAR and SBS  files for those who want to study it in detail. You can get it on Gumroad. The node is also available on Substance Share.