Hey, if you just want to know the option within the game than try different quality settings for the Mesh Quality after you set the Overall Quality to Custom. If you actually want to change the distance of the different LODs per game object for your map than you have to enter the world editor. Go to the Shape Editor and choose the object you want to modify. In the details tab you can set the size per LOD. Check out the wiki page about exporting static objects. There's a nice overview.
