The Fractal Size dialog box is where you can specify the pixel dimensions of a fractal displayed in a fractal window. You'll want to set a fractal to a specific size if you will be creating a zoom or iteration movie or saving the fractal as desktop wallpaper.
The "OpenGL sizes" are image sizes that work particularly well with the OpenGL version of the Zoom Movie player. You may get better OpenGL performance on Zoom Movie playback at these sizes.
The Desktop option sets the fractal size to your screen size, maximizing Fractal eXtreme. Use this setting when you want to save a fractal as desktop wallpaper that fills the entire desktop.
Note: If you use the desktop setting, both the Fractal eXtreme window and current fractal window enlarge to their maximum sizes.
To open the Fractal Size dialog box: On the Options menu, click Fractal Size.