This great and often requested feature is available since version 3.18.0.0.
Scaling or zooming works intuitively and separately for each video window:
▪Click the left mouse button and select the desired rectangular area on the video by dragging your mouse.
The selected area will immediately be scaled to fit the size of the window:
▪To return to full view, left-click into the video or select an area smaller than 50 by 50 pixels.
Zooming works both during pause and during playback.