Giving customers the ability to input a specific size.

I was wondering if there is a way to give customers the ability to input a specific size for a banner, in feet, in a product page, and it would calculate the total price based on the price contributors will set. So, for example, a customer wants a 4x5 feet banner and the price of the banner is $12 per square foot, so the price of the banner when the customer adds that product in the checkout would be $240.

If that is way too complicated, any suggestions or advice on how to approach this is welcomed.

Squarespace does not offer the ability to type in a size, but you can offer a predetermined list of size options by using variants.