I think you guys expect a volume of people to be involved that is far greater than is actually going to be involved.

I was thinking of plots 500 km x 500 km with an allowance for people to claim multiple plots. France was something of an anomaly in terms of size in the medieval period, most countries were much smaller, think of the Untied Kingdom, and then break it off in to Scotland, Wales, England, and Ireland. Even France was carved up pretty significantly, you had Burgundy, England, Savoy, I think Brittany and all sorts of holdings to the East. Generally though what I use to base my conception of country size in such time periods is Italy.

As for the grid to use for plots I think we should do an expanding grid. Given our equirectangular projection this would mean plots stay the same height through out but that they expand horizontally as we move up the latitudes. Here is my example:
Click image for larger version.  Name: Grid example for net1.png  Views: 39  Size: 33.3 KB  ID: 66489
The size of the lateral expansion is set to 500 km at that latitude from the middle of the height. This means the edge closer to the equator will be bigger than 500 km, and the edge farther from the equator will be smaller. So all plots here are roughly 500x500 km. Above 60 degrees of latitude we should have pole end world maps and we will divide them accordingly.

600 miles by 600 miles is truly immense, way bigger than I was envisioning. I think that it was geared to "fill in" the entire world quickly, the way I'm approaching this project is that we have a world and setting that people can come in and fiddle around with and then take a break or whatever. The aim isn't really to "complete" the world for me.

Also in this example is how an active plot would block plots from other people, any of the white plots bordering those areas would be available to work. If the person was unconcerned about working with others than he could leave the red areas open for mapping as well. I think though the default should be set to blocking out those areas. This allows people to come in comfortably map what they feel like and not have to worry about coordinating with others etc. It's far more relaxed. It also allows us to be very vague with the world map.