The preview images that go in the kits should be 600x600 to keep the file size down. I know when I save files at 3600x3600 they are huge. e.g. one page I did was 185kb at 600x600, but 7.7mb for the full size. We can change it for you so you don't have to reload it, but for next time (and for everyone else) please have the preview images in your zip files 600x600.
Thanks!
I hope you had fun designing!