I think I know the answer: If this were possible then people would go buy a frame and then browse for prints of a certain size to fit that frame. Art.com must feel this will result in a loss in sales of their outrageously priced frames and custom framing services. They’re probably not making a ton on prints by themselves so they are banking on the revenue from the frames. Therefore I feel their website is designed specifically to detract from people browsing for prints of a specific size to fit an existing frame.
Art.com is a located locally to me right here in Raleigh. I do think they have dominated the on-line decorative print store fronts so I find any alternate answer hard to believe. There’s no way their site designers can’t find a way to query all the prints of a certain size for a return result. Instead they require you to specify an artist’s name or title of a print before you can narrow down the size of the prints you want to see.
I might just buy the new prints for my office elsewhere due to this intentional inconvenience.