When you display anything you get from your variant that is size, and it is in cm, just use Liquid to convert cm to inches.
There are math filters in Shopify Liquid for this. So you can easily divide or multiply for presentation purposes. Except for Checkout. That is where it might suck to be you. You don't get to fiddle there.
Otherwise, if you are happy just converting, write a little script for your store that goes through your products. For each variant, find the size and convert from cm to inches. That is a simple code exercise you can pass off on a noobie coder no trouble.