The user interface leads to the first restriction: as it is using space on the screen to show the window controls, window frames etc., the interface steals area for documents. Thus, in any case the graphical images need to be smaller in size than the particular computer's screen resolution.
The screen resolution itself is a problem: a regular Macintosh computer has a screen resolution of
640x480 pixels (see page 
for explanation), but depending on the monitor and model
it might as well have 800x600 pixels. The same holds for personal computers running the Windows
system: screen resolutions from 320x200 up to 1024x768 occur. Graphical workstations like Silicon
Graphics have a minimal resolution of 1280x1024 and thereby provide a huge area for image presentation.
But from the view of a designer, no assumptions about the computers used to view a designed graphical
image can be made. The only choice insuring a design will show up on a computer screen in the
appropriate way is to limit its size to a reasonable assumption about the reader's computer systems 
.
I recommend assuming that every computer used for multimedia applications supports at least 640x480 pixels of graphics resolution.