Thanks for redirecting the focus cypeg. :)
I like talking about the map, so I'll give you my thoughts on the issues you bring up. There's not necessarily a right and wrong way to design it, nor do I think I always am right on how to balance it, but I have opinions and I'll bore you with them. ;)
You're already onto the issue with sea zone size and the pros and cons of big sea zones. Bigger sea zones makes for easier defending of unified continents while smaller isolate the continents from each other. I went for big sea zones because I wanted cross-continental interaction, which I believe adds to game play and increases options for players. It might cause a few headaches, but hey, it's not a feel-good game. :) The main trouble is stalemate as I see it.
I was looking a bit to the standard map on the sea zone issue, and I would like to say that the North Sea is one solution to the problem of sea stalemate. The North Sea is a big province, but it is surrounded by a bunch of small provinces (if you check the original plans for the standard Diplomacy map, it looked a bit different I believe). This means that you can bring in a lot of support for a unit in the North Sea, but the enemy can also bring in a lot of units to attack it. Thus the defense falls against a stronger enemy. The worst thing you can have is big provinces border each other, as that means you might defend with 8 from one side and only attack with 2-3 from the other. This line of thinking is rather new though and might not reflect everywhere on the map today (but NAO is a result of it). It might not be possible to use everywhere either, and it does not remove all the stalemate lines, but maybe it removes much of the problem they create.With this approach I believe stalemate lines will rarely be around in the early and mid-game. They appear when someone essentially owns a continent. The recommended solo number is 37 (out of 107; and you need to be at least 2 ahead of the runner up), so when all neighboring continents are defended we are talking about the last handful before a victory, which is quite normal. Playing to 50%+1 might be another story altogether, but world maps take quite long as it is so people are usually happy by 37.
USA tends to fight UK over Vancouver first, otherwise I have not noticed any necessary immediate conflicts of USA (Manila mostly falls to Japan, but the idea is not that they should try to hold it very often). The USA-Mexican relations though seem to be necessarily hostile in all games, and they fight happily after two years or so, always. I'm hoping to make the relation more dynamic and less predictable. And yeah, Germany has New Guinea. A real game maker, that fleet :p
Europe do stand for a big chunk of the map, but it is after all a 1914 map. Europe hosts 1/3 of the centers perhaps, but 1/2 of the powers so competition is fierce (remember the colonial build rule means you still are quite screwed if you lose your home centers). True that they have more neutrals than Asia, which is partly due to how the world looked like 1914, but in return there are plenty colonial outposts to grab that the Europeans have a hard time to defend. Japan is however a bit slow, so I'm looking at how to promote Japan without making it impossible to take for a successfull attacker.
Australia sure looks big, being only one province. What I would worry about splitting it up is that since it is all British nothing would happen there, so it'd basically just be a bunch of guys catching crocs and drinking beer.
A big and tricky question is how you promote Austiran opportunities. I've done alot to direct other powers attention elsewhere, but I'm not sure if I'm closing up on my one in every twenty solo objective which I have as lower limit for how often any nation should win. Europe can't be changed so it's really not easy.