How would I go about creating a game in SDL that is HD? Meaning 720p. Should I specify screen height as 720 and screen width as 1280? Is it something entirely different?

How would I go about creating a game in SDL that is HD? Meaning 720p. Should I specify screen height as 720 and screen width as 1280? Is it something entirely different?

Yes, that is all there is to it, the HD term is normally not used for PC games though as resolutions for PC games have been higher than 720p for a significant amount of time. (PC montiors capable of 3840x2400 have been available since 2001 and the primary reason why most PC monitors today settle for 1080p is because LCDs look best at their native resolution or half/quarter/etc of it, (and going up to 3840x2160 is awfully expensive and to watch a 1080p(most bluray movies use this resolution) movie in fullscreen any resolution between those two will look worse or not use the full screen space)