I agree, but its not that easy.
TMDb has small postcard size fanarts available for browsing purposes, they are all the same size and would give no useful information as to the fullsize image size.
The only way to do this would be to actually download the fullsize version of each fanarts and display that instead of the previews, this would be a big waste of TMDb's bandwidth, aswell as very slow, especially for movies with losts of fanart.
I may add somethink akin to that of the poster browser, where double clicking an image displays the single image with the full resoulution, I'm undecided at the moment.
Perhaps it would be more useful to petition the TMDb developers to add a fullresolution tag to the API