In terms of how much fiber to spin to have 200 yards, you'll need to look at "yards per pound" measures relative to "wraps per inch" measures. Now, this can vary _alot_ depending on the relative weight of the wool -- this gets into terms like "grist" that make me cringe ;-) but just realize that if you squeeze the air out of your yarn, it will be denser and thus heavier for a given length than yarn spun to the same thickness but unsqueezed.
That is why, when you try to make yarn charts on WPI relative to YPP, you get such wide YPP ranges. There's one other online chart, here: http://www.spinderellas.com/patterns/yarnchart.html
Using the Yarn Standards from the CYCA, and the wraps per inch from the KnitPicks catalog, along with the above chart, this is a mapping of standard yarn thicknesses to yarn weights (remember I warned you, the YPP ranges are huge!)
Sorry for the gap ... or maybe this is blogger's way of interjecting a "spoiler alert, eh?"
6 or less
400 or less
So ... if by medium weight you mean the CYCA #4 medium, then you're talking a worsted weight yarn of about 9-11 WPI, which is about 900-500 yards per pound, so you'd want anywhere from 4 to just under 8 ounces of fiber. You'd need less for spinning "woolen" or airy, unsqueezed yarn and more fiber if you are spinning "worsted" or compressed, smooth/tight yarn.
(based on a post by me on spinning_camp, 23May07)