This Light-O-Rama Visualizer dialog is accessed from the Simulation Properties dialog box after pressing the Change Size button. This dialog allows you to change the size of the background, as well as resize and reposition any existing fixtures.
The Visualizer's Change Simulation Size dialog
Original Size
Displays the current size of the simulation in pixels.
New Size
The updated size you wish to use for the simulation.
Maintain Aspect Ratio
When checked, any change to either the height or width will automatically change the other field proportionally. For example, if the original size of your simulation was 800 by 600 - a ratio of 4 to 3 - changing the width to 1024 will automatically change the height to 768, maintaining the ratio of 4 to 3. If this is unchecked, you can change the fields independently.