How can I find out the dimensions/resolution of a downloaded video file?
MX Player provides a way to check video resolution. All you have to do is install and open MX Player, touch hold the video file until you see some options and select properties. There you can find resolution listed as well as other codec information.

Besides, VLC for Android also shows the resolution. Install and open the app, left-swipe and choose Video option. There you can find all the videos scanned by VLC with resolution already listed on them. However, if you choose Directories option, then you can't see the resolution.