It depends whether size is a custom field, variation or an attribute.
In case of a custom field, you need to create two coupons (one for each %) and use the “Custom Fields”-option on the usage restrictions tab.
In case of a variation, you need to create two coupons (one for each %) and select the variations in the “products”-field on the usage restrictions tab. If you have many products then this will be very cumbersome.
In case of attribute, this is not supported out-of-the-box…
There I have created a product text field. Unfortunately this is not found under the custom fields (Restriction Tab).
This text field gets filled by a datepicker with a date (format dd.mm.yyyy). Now I would like to use a different coupon depending on the date.