While working on my aircraft kit I needed a huge terrain to fly around; I decided to create more terrains and make it available in Asset store. I generally take a while and 'touch' the terrain features and textures by hand, for a better quality so it should take about 2 weeks before its ready for prime time. The aircraft is about 3 times smaller than the reconnaissance balloon, for size reference. The grassy terrain for instance is 6000x6000x 450 (height). Heightmaps will be provided in 4096 (which you'll never need in unity) so you can adjust the terrain size and detail from there. For these sizes, the color map is preferably kept at 4096 as well. iOS/Android version might be available as well, with the terrain exported as models since Unity does not support terrains on mobile.