Hi,
Well, it's normal that the quantity is taken into account for the options.
For example on that page:
demo.hikashop.com/index.php/en/hikashop/...ct-page/with-options
If you order two computers, it's normal that the options are counted twice.
What I would recommend is to check the quantity input method setting of the product to not have it, and then instead create a characteristic with values the customer will be able to choose from for his product. Then, in the product, you can select that characteristic, and under the variants tab, you can add the different quantities as variants and enter the price for each variant.
That way you will have (Length x Width x Price of the selected quantityb) + 10