I was wondering if there was a way to make a very long map, or is there a set size?

You can make them bigger.

There's possibly some limit, but chances are, your computer won't be able to run the map any more due to lag before you hit a limit like that.

how hard would it be to make just a long plain dirt map?

Pretty easy, but I have to go for a little bit.

Look in the .ini files for the scenes.

my test map is 6000x2000 and the game is quite slow. I think 4000x1500 maps are quite good in this point of view.