The Smart Resize feature comes with a useful option that allows you to save all your custom sizes not just in the Resize list but also in the Generator preset list, and all you have to do is add your custom size and click on the Add size button.
Managing your custom sizes
The best thing here, is that all of the custom sizes are automatically imported in both the Editor and Generator and you can even rename them to keep them nice and organized as size templates.
If, after a while, you start having too many old custom sizes in your list, no worries. You can easily delete them from the sizing section and add new ones whenever you want.