Maximum resolution for standard definition video is 720 x 576 (PAL), or 720 x 480 (NTSC). You’ll find that some people will convert standard definition wide screen anamorphic video found in dvds (non-square pixels) to a larger square pixel format, like an 852 x 480 avi file, but the quality does not go up when doing so, and it won’t play on a standard dvd player which has limits on resolution size.
Anything above the SD resolutions can be described as HD, though most of us would define 720p as the start of high definition resolutions. 720p is 1280 x 720 resolution.
There were some intermediate resolutions in certain countries, like 576p(50fps), but they have been replaced in tv transmissions by 720p or 1080i. 576p was also called EDTV.
Most of the odd sizes you see in “high def” files are due to people cutting out the black bars on wide screen video. They were probably from 1080p blu ray sources, then converted to something smaller to make them easier to download.
And just because they are larger resolution than standard video doesn’t make them high quality. If the bitrate is reduced too far, to make the files smaller, the quality of the video can become quite poor, even when using a fine codec like H264.
