Chris,
Regular figures have several properties that allow you to control their size when converted to an image, and should work with pdf files too. Not sure if this is valid for UIFigures.
PaperPosition: standard 4-number position vector
PaperOrientation: "portrait" or "landscape"
PaperPositionMode: controls the position of the image on the page. when adjust the above two properties, you may also need to set this to "manual"