It seems to be 2 MB.
But actually I do not care about it and upload 50 MB videos successfully (maybe Telegram recodes them itself).
1.
2.
Checking the video is square, under 10s, smaller than 2MB and have smaller size of 800x800 pixel
If possible mute the video that has sound
github.com/telegramdesktop/tdesktop/issues/8304#issuecomment-778014437