From OpenSCADAWiki
For any frames in the visual editor available only visually changing the geometry, what you can do by placing the mouse cursor to right and/or bottom edges of the frame and next move the edge whether by mouse, holding the left button, or by keyboard, pressing the arrow keys — the arrow keys clicking cause of the corresponding size changing at 5 pixels on 1x scale and holding the Shift key you get 1 pixel. You can also change the frame scaling in the same way but holding the key Ctrl or toggling of the size mode on the status panel right.