The simplest way would be to go into layout view, doubleclick on the popover button and set the popover size manually.
If you are looking for a dynamically sized popover then, as far as I know, it can't be done (though you can do a workaround with multiple layouts and/or several objects of different sizes with different hide conditions).
I would guess that the square white field below the popover is an interactive container field. Unfortunately, FileMaker does not allow popovers to extend across web viewers and interactive container fields. If you are using those fields to display images, use the inspector to optimize them for images and then your popover will no longer be "clipped" by the container field.
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Perhaps, if you set the popover to pop from the bottom, instead of the left side. As long as it does not run into what Phil, above has described that clips the popover, may have to change the popover size.
Thank you for the suggestion: I had the same problem and it worked for me.
I found that that was not a bug: the problem was due to an overlapping between the popover and an interactive container field. That's not a problem on OS X, but it is on Windows, as stated by FileMaker in the Help page at "Adding a Popover", under Notes (second bullet).
Windows: Popovers cannot display in front of web viewers or interactive containers in . To avoid overlapping these objects, a popover will shrink, if possible, or display behind these objects.