Today a big new map (3040×1600) of Russia created by AEO was added to the Simutrans maps website. If you use this map be sure to set the water level to -2. The recommended number of rivers is 1024, the maximum length of rivers is also 1024.
Further, a map created by Sindor was added in two sizes (256×256 and 1025×1025). Use water level -6 to get the intended map. As nobody seems to know if this map actually represents a real part of the world, it can be found in the fantasy maps section. Please make a comment if you know which interesting part of the world this might be.