And pfm beat me to it with the Jynx and Hoothoot Character Rares for smallest, those came to mind since I thought they were pretty small in their full art artworks when I added them to my Supporters/CHR collection.
PS: Cameos shouldn’t count for this thread imho, since some are astronomically small and barely larger than a pixel. For example: the Wingull and Drifloon are pretty dang small here:
Worse still, there is no way to fix the Mewtwo with the imagination. Zoom in and you’d leave the bottom part out which would be just as bad. Replace it with one of the promo arts and it no longer would feel like a Base card. And this is without even mentioning the 60HP and how the non-shadowless unlimited art leaves out the dark spiral in the background which is the best thing about the entire card.