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…