Pricing based on the printing size

We are charging customers based on the size of the print which is $5/sqft, regardless of the color, text, and layers.

I've already purchased the Plus addon and tried to use the price by cm2, but the problem is that when users resizing the canvas size in Dynamic view, let's say changing to 5cm by 5cm, the canvas will be too small and because all the designing tools are inside the canvas there is no way to design the item.

I was wondering if we could put the tools out of the canvas, or keep the canvas in large size like 1000px by 800px and let the users to create their own printable area box and then charging based on the size of the printable area.