Zoom effect is explained here
. It looks, we have to update the details there.
When you apply zoom effect, you should see an additional block in your project above the video you are going to zoom. When that additional block is selected, you can change the size and position of the area to be zoomed in the preview area. The rectangle can be of any size and take any position in the frame.
In case you lost that rectangle for zoom in the preview area, just select the original video block in the project, then select the zoom block again.