Koltomic.
Well 4096x2048 was meant because it's a power of two. Dan said textures which are a power of two look or load better in Ubox so I guess there are some general benefits of using that. 5010x2505 seems pointless imo, might as well be 5000x2500 so it's easier. This planet doesn't have to be (exactly) the same size as Earth.
The closest other powers of 2 would be 8192x4096, which seems too big imo, or 2048x1024, which seems too small.
I really like the 2:1 aspect of the map, though. This should make the reason fairly obvious. The green is a line which would follow the height of the image. The red line would follow the width: