Since you have climates ranging from glacial to scorching, it seems pretty obvious that it's half a planet
Even if this is true, it doesn't mean the planet is necessarily the size of Earth.
I think the issue here is that there are multiple scales being envisioned, and the civ sprawl is the first feature that measures from the inside out. The size of the villages and their farmland is based on the 1x1 creature size, so that in adventure mode you can walk around and feel like things are mostly to scale to your adventurer.
The other way of measuring scale is from the outside in. A medium map has oceans, major rivers, mountain ranges. What is its footprint supposed to be? A hundred miles? A thousand?
I'm sure someone has done this math before, but I can't find it. Let's just hypothesize that a 1x1 tile at the lowest level of play is a square meter. So a 1x1 local tile is 48 meters across, and a 16x16 local tile (1x1 region tile) is 768m across... so a medium worldgen is 129x129 of those, yes? That's 99 km on each side.
Our perception in previous versions on embark is that we are creating a bigger world, one that is much larger than 99 km or 62 miles across. Our whole dwarven civilization might span a 3x5 area, which would be less than five miles from one end to the other. I don't think that's how we treat the game from the DF embark view. We abstract the world map into being a world much larger than the game thinks it is because we see oceans, glaciers, mountain ranges, and lakes that we imagine being tens or hundreds of miles across. Sprawl forces DF to exist in Adventure Mode's scale, and that kind of breaks it, at least from the embark screen.
Unfortunately, this is the first feature that really highlights just how difficult it's going to be to create two games in one. Adventure Mode and Dwarf Fortress exist in two different realities, and I tend to think it will get more complicated from here to keep the two together as a unit. Sprawl creates farms and villages that are to scale with your adventurer, but they are ugly behemoths when considering your new fortress, and they're *everywhere*.
I think sprawl is an important feature for world immersion in adventure mode, and I applaud the effort made to simulate the amount of land required to feed thousands of people (something most other games get totally wrong). I just don't think it can coexist with fortress mode in its current state. A decision may have to be made on which scale makes more sense for this game. If the 99 km region/island attitude is preferred, then civilizations should do most of their work off-map. In a fully imagined world (thousands of miles across), one that we are taking a narrow look at (a hundred miles across), we may see ten settlements instead of two hundred. Terrain features would take up more space. And on and on.