Based on my experience, selling the work as a print should be the same price at any size IF* The image is done as a series, One number of say 100 total. What size the customer prefers should be non important- if the customer wants the print they pay 750. Most credible artists increase price related to factors of their name and what its worth and perhaps how many copies have been sold of said image up to date. Or they just pick a price they feel their work is worth. If an image is unique + only one copy is made then the price should be a-lot higher!

If I have lots of publications, and my images are already sought after, then I should command more dollars- for any image- because that is how other artists price thier work. If one prices the way that you described, that says amatuer. That said I do have images that print better at certain size versus a range of sizes, therefore I only offer one size.