1. Go to:
Fixed size 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 fixed size product Resizable parameter should be set to "No". You can switch on/off rulers if you want to:
The product without rulers:
The 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"):
See the result:
8. Print area dimensions you could see on rulers points:
Mind if you want to change dimensions, for better looking you should keep respective aspect ratio of your .svg product image.