"HD" simply means high resolution format...

Now it depends what you want to do. You can watch 720p movies fine, but 1080p needs more resolution, so you will need a 24" 1920x1200 screen for that. You will likely want an HDCP compatible monitor with HDMI for that, which my Sceptre x22 has, and I believe is one of the only 22" screens that does.