I sell A4 & A3 prints too and the way I do it is with separate listings. You can offer variations on the same product by asking people to leave a comment with their order telling you their preference, but that doesn’t work in this situation where the price needs to be different for each. (It works better for things like selling the same product in two different colours.)
If you’re worried about the listing fees adding up, one option to consider would be whether it’s worth getting a Plus account - that way you pay a fixed fee for unlimited listings. If you’ve just started out you might want to test the waters before committing to that, but it could be something to bear in mind.