Selling art with Drupal Commerce - how to approach?
Products you sell are not nodes in Drupal commerce, but are their own entity where you can define product related stuff, like SKU, price etc.

You probably will need two product types, one for originals and one for reproductions.

You will need to create a product reference field on a node or something else in order to display the add to cart form along with product details, this can be done on your work of art content type.

In order to select size, the easiest solution will be to create on product per size, then reference all of them and commerce will do the rest.