Learn about resizing the Render Window in VRED.
Video captions: To adjust your render window size settings, please open the Window menu. The default value, Dynamic Resolution, indicates that your render window size depends on your user interface layout and your scene resolution.
To set your window size to a fixed resolution, choose from different presets. Now your render window remains independent of your GUI modules.
To customize your fixed resolution, open the preferences and adjust the size in Render Options.