(FULL-SCREEN) makes your 'CONSOLE window as big as possible, reserving the last line for garbage collecting information, while (SPLIT-SCREEN n|) makes the 'CONSOLE window n| lines big, at the bottom of the screen, which allows you to act interactively on the graphic screen without scrolling up what you've drawn.
Moreover, you can drive it from within your Scheme programs and customize it. By default, it uses BRIEF's key sequences, which are very convenient with traditional extended keyboards.