I had a few more thoughts on game size again. Since I already had a few thoughts a while back I'll just http://www.gamesas.com/bgsforums/index.php?s=&showtopic=1032326&view=findpost&p=14977249
But I've been thinking about the actual size of the map, in the old thread I mentioned "It should take about one week in game time to fast travel from one end of the map to the other if you follow the main roads and take regular breaks". Basically walking normal speed (Not running), stopping every night and taking breaks amounting to 8 hours of rest and only following the save main roads without going cross country or taking side paths, that would take about a week to cross the map.
I did a little bit of math, this would amount to walking about 100km (~63 miles) with a regular walking speed of 6km/h (~3.75 mi/h), however you also have to keep in mind that the streets wouldn't be straight but wind through the country, go uphill and downhill and have zones that take more time to cross so my idea on total size was about 60 x 50 km in size (37.5 x 31.24 mi) which IS still a reasonable size seeing games like "Just Cause" and "FUEL".
However it doesn't have to be that big really. As mentioned in the old post, if you take one of the islands from "GTA: SanAndreas" the area would feel about right for a "region". This is mainly based on the northernmost desert part of SA because it doesn't feel too large and inflated but also not too small and like it was just squeezed in there.
So take a climate region (like the swampland, mountains, golden coast... in Oblivion) and make it about that size, that way they are not ridiculously huge but also don't feel as cramped together as they did in Oblivion sometimes.
Plus the bigger they are the more it allows a transition zone in between and, what actually is important, http://tvtropes.org/pmwiki/pmwiki.php/Main/Sceneryporm. I remember that one fake screenshot of TESV with the mountain in the background, that wouldn't necessarily serve much a purpose but it looks more convincing.
Such things are important too but need to be done right, if I remember the cheese melt mountains in Oblivion they weren't really convincing at all.
So in conclusion, what would I say. Of course I'd say "bigger is better" since it simply allows for more but I know it can get hard fast.
My recommendation for size, something between 20 x 15 km and the mentioned 60 x 50 km, drastically different numbers but both still somewhat reasonable.
As for the design problems I'd again go with the mentioned "Unique Landscapes" method, roughly design the complete area, roughly place the areas, determine their size and the total area. Then split into teams, each taking on a unique area and designing it. Then put the pieces together and "blend the edges".