Search by Product Attributes

For a new shopperpress website I’m building, I want people to be able to search by size and colour which are attributes of the product.

How can I add these attributes to be selected in the advanced search? I tried adding them via custom fields but it gives all the size attributes as one selectable option (e.g. instead of size “6” and “8 “as unique options, it has “6,8” as one option).