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)