Restricting image size to maximum 1200 wide and 1500 high is going to handcuff my style badly. I and many have got used to being allowed images up to at least 1800 wide x 1000 high, i.e. the biggest that (plus edging) can fit into a modern wide-format desktop computer screen.

Those new enormous thumbnails mean longer download times and far fewer thumbnails on a screenful when quickly looking through a gallery. The idea of a thumbnail is to be compact, like the nails on my thumbs that I can see as I type.