1. Go to:
Resizable product looks like this:
2. Click "Edit"
3. In edit mode click "Duplicate" to make a copy of this default product
4. Now you're editing duplicated product. Go to "Designer" menu item
5. Here you can choose thumbnail image (should be 110x110 px JPG, PNG, SVG)
You could see it in designer on "Select product" panel
6. For resizable product Resizable parameter should be set to "Yes". You can switch on/off rulers if you want to but it's better to keep rulers on.
Product with rulers:
7. You can set your editable area coordinates (to see editable area borders on design set the "Hide Editable Area Border" parameter to "No"):
8. Print area dimensions you could see and change on select product tab. Also you could see them on rulers points:
9. You can set up minimal/maximal values for width and height of resizable product: