For the total population of an an average village, it's easy enough to calculate. The Domesday book has about 13,000 entries for an area covering maybe 100,000 km2 (England minus various parts) and an estimated population of about 1.5 million, iirc.
Frisicus had a good description on the difference between hamlets and villages, but I think his numbers are a little low. The Domesday book means one settlement for every 8 km2 if they are evenly spread out (which they aren't) with an average size of about 100 people. Eight km2 means that each settlement has a "catchment" radius of about 1.5 km, meaning that settlements are about 3 km apart if they were evenly spread out, maybe 2 km if they are distributed realistically (along river valleys and roads, with wooded hills in between).
Now, a community of 100 people probably can't support most of the institutions that rural folks will need, if not on a daily basis than on a weekly basis. Mills, castles, churches and/or Friday mosques, markets, etc will probably be located in centralized, larger villages or market towns. In Sicily I'd guess that there were about 150 of these for an area of 22,000 km2. In Domesday-covered England this would mean about 800 settlements, which is probably-not-too-coincidentally close to the number of burroughs that ultimately received a license to operate a market.
So, instead of thinking of villages, think of a "rural economic unit" composed of one large village surrounded by a number of hamlets and small farms. Travel time (a few hours in each direction with a cart, probably) determines the size of the unit, crop yield determines the population per km2, and many, many other factors -- legal, military, economic -- determine the distribution between village and hamlet.
To pull a number out of a hat for England in 1100, let's say each unit represents 120km2. Each hamlet has 20 households, or 100 people each x 15 = 1500 per "unit", and each market village has 100 households, or 500 people, for a total of 2000. Sicily in contrast would have had a much higher total population (it was the grain-basked of the Med for a while) but because of the constant warfare and other reasons, a higher concentration in the hilltop towns. So in Sicily maybe 2500 people live in the towns, and 2500 in each of a dozen small farms, to come up with some very out-of-the-blue answers.
I thought this website did a pretty good job of doing speculative maps for difference village sizes:
http://www.aedificium.org/Maps/LocalMaps.html
I might have done the numbers wrong, but that's the gist of my estimations.
If you really want, I took some very detailed notes from the Domesday book on Yorkshire for the (now on hold) Northumbria mod. I can post those if you like.
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I know that this doesn't address the main part of your question. I don't think that half the population would be fighting-age males -- add in children and the elderly, and you could go as low as 25 or 30 percent. Cirdan's point is very relevant about the type of labor involved affecting what would happen if the males disappeared. There could be huge dislocations of population in the Medieval era that would never have shown up in the aristocracy-centered chronicles at all.
For some very basic, very rough figures, I seem to recall that the conventional wisdom is that pre-industrial societies can supply a maximum of about 10% of the population to the military or to other mobilizations, although I might be wrong about that, and this could not be maintained for very long at all. The largely agricultural Tsarist Russia only mobilized about 7 percent of its population for the whole of WW1.
Labor obviously was a relatively scarce commodity in the medieval era, or they wouldn't have introduced serfdom. However, I suspect resource rather than labor shortages were the ultimate cap of mass mobilization, however. For medieval campaigns, peasants weren't mobilized en masse very often because they were difficult to feed and they weren't much use in battle. This didn't mean that harvests didn't affect the pace of campaigns -- peasants would want to be home at harvest time because it was important to them as individuals, even if there wasn't a critical labor shortage overall.
France in the Revolutionary and Napoleonic wars might be a better model to answer your question, as better roads and canals meant that you could move and feed larger armies, and the use of the musket meant that large amounts of untrained troops were more militarily useful.