When you go into the edit mode for your terrain you can select the size and resolution of the terrain first, depending on what detail you want you'd have to go higher. Generally a a grid of 512 is good. Then when you export you can also choose the size of the map and format. I usually go with tiff since all detail is retained. And don't forget, terrains and lattices are not only for landscapes. Using greyscale images you can use it as a modelling tool. There are many good tutorials on this but I can't find them offhand. Have a look at Lannie's links at 3D Model world (link on the right) Also have a look here http://cgcreators.com/yuji/ This guy models everything with terrains! Download a few, take them apart and you'll learn a lot!