The default sizes seem really dated; I'm proposing that they be updated as follows:
Small: 854x480 (from VGA to WVGA)
Medium: 1280x720 (from SVGA to HD 720)
Large: 1920x1080 (from XGA to HD 1080)
Handheld: 480x320 (from QVGA to HVGA)
Or maybe Medium should be 1366x768 (WXGA) instead...please comment if you have an opinion.