Unfortunately only a few Plex client types support zooming the picture. Namely Plex for Desktop and Plex HTPC.
And even these need to be taught first to do that:
The methods described in this post are not available in other Plex clients apart from “Plex for Windows/Mac/Linux” and “Plex HTPC”
in this post:
Pausing and stepping through a video, frame by frame
Video zoom/crop
Fine tune vertical position of subtitles
The internal AV engine in these players (called ‘MPV’) supports a lot of features which are not available in the Plex user interface.
Some of these features can be used, nonetheless by “circumventing” the Plex user interface and assigning…