Depends on the source of the video, I think. Some of mine show under the "More Info" section, and some don't. Playing them and getting the info from the Player is probably the best way.
As MacOS do not support .mkv files in the base system (like mp4 and so on) you will not see resolution in the info box. If you open the video in a videoplayer (like vlc, mplayer) you will be able to look in at the info inside the app for the resolution.
may not work for everything, though. And it uses spotlight, so it needs to newly index the file and record the"Dimensions" before they can be displayed in the finder. (It's a one time hit, though)