The ad sizes will change according to what screen resolution you set the systems to.
In the 1024 x 768 mode the Left side of the screen uses jpg images at 960 x 615 in pixels and any other size will be stretched or reduced to fit. The right side uses gif files only at 320 x 615 and depending on the file some will animate, but not all.
When you use the Custom Screen Sizing mode the sizes will be shown to you on the Desktop control panel software and when pressing the letter “h” on the keyboard connected to the ad server. More about this here