If you want flat land you can use any seed you want, but raise the X and/or Y value to a very high number (at least 9 digits long). The farther you get away from 0,0 as a starting point for your map a strange thing happens -- it's as if the game is using a bigger and bigger brush to draw the landscape. This "brush" is always square and the farther from 0,0 you get the larger this square brush size will be. Eventually you will see all the land features with square shapes that are 10 or more tiles on a side: square lakes, square surface clay, square hills.
In addition to the surface land features all being big square shapes, all the underground caverns will also be based on squares as well.
It can be fun to mess around with for map generation. It is the best way I know of to get a massive amount of water on your map if you want it.
Just remember that whatever numbers you use on the preview screen for X and Y, be sure to double those numbers exactly when you are ready to hit the "generate map" button.
You can see an example here
from a kingdom I made last year.