I have been experimenting with large layouts recently. My 14 year old son wants to make a 2d minecraft-like game. So, we have been playing around with auto-generating terrain. We are using a large tilemap object that is 1000 blocks wide and 64 blocks tall (tiles are 32x32). It has the solid behavior - so anything you can build, you can walk on. There is another tilemap behind the main one (without the solid behavior) so that when you dig into the ground you don't see the background, and then there is another tilemap in front that hides what is in the ground unless you are close to it, or leave torches to illuminate the area.
I am very happy with performance on computers, but mobile is terrible. Generating the terrain is really fast! Our tile set doesn't have very much variety yet, but we are planning to add diversity by expanding it to represent different biomes.
You can dig blocks out of the ground, but you can't build on top of the ground yet. Press A and D to walk, W to jump (Space bar for ridiculous jump). Hold Shift down to speed walk. Left mouse button to attack, right mouse button to dig. Press T to leave a torch.
Press F to teleport to the left side of the map, G to go to the middle of the map, H to go to the right side of map.
Press ESC to re-generate the terrain. (Most of the graphics are just for testing purposes)
Try it out at: http://www.rieperts.com/games/Goblin/index.html