So basically, when a customer goes onto my sticker website. They will see a custom size option that allows them to input a size of their choice on the width and length in mm. It will then show them the price for this on the next page. Is this possible? Like this. Measurement Price Calculator - WooCommerce
Have a look at the last comment in this other forum post and see if Yisrael’s linked example can help you get started too.