You do this with quotas, though I would suspect doing this with Windows in a non-domain environment would likely end up being more trouble than it's worth.
Either way, try typing in something like "Windows quota" into Google and see what pops up. Should be enough to get you started.
When I say "size" I mean the alotted hard drive space a certain folder has, not the window on the screen. Reason is because I want to make a network drive for another computer that is only allowed 100 mb. How do I do this?