This is logistically impossible in the game. The best you can do is a six player game. Their just aren't enough tiles, borders, and numbers to build a board that would allow 10 to 12 people to play. I suppose you could try combining several sets, but then you're going to run into issues with number tile balance, hex tile balance, etc. Also, with that many people trying to play the game, the game is going to take an exceptional amount of time because of how many people will be taking turns, and you'll have people getting bored quickly. It will also reduce the total number of turns until someone wins because with that many people rolling the dice and getting income, income rates will be artifiicially inflated. In my opinion it just won't work that well.

The largest scenario in the game is "Greater Cataan" in the Seafarers rulebook. It requires two basic sets of Settlers with the 5/6 player expansions, and a set of Seafarers with the 5/6 player expansion. I've played it before, and setting it up and playing it is difficult due to it's size.

If you want to talk actual "size," I heard there was a giant game of catan at a gencon a few years back, with road segments the size of an arm or leg or something. Supposedly, one could go to the second floor and look over the railing of a balcony to see the board from a top-down view.

The "giant settlers" game I played at gencon the following year was not that big at all. It was about 3 times the size of a normal game. It makes me wonder if the stories I heard about the previous year's gencon were true.

The biggest I've played was the 6-player expansion, using two basic sets. It was pretty fun and kinda overwhelming since it was only the 3rd time I'd played.

My favorite is playing a 4-player game using the basic set plus Seafarers and Cities and Knights. There's alot of action playing that way!