This is a difficult question to answer. Quite a number of variables involved.
Video cards process textures using filtering to get the texture to look right at the given distance from the viewer/camera in a 3D environment.
In other words, unless you are right on top of the texture, you will never see the texture in the original design format as the video card is always scaling/filtering the texture down to fit the given distance.
And that is the rub. The quality of the texture is strictly dependent on the video card's filter algorithm. Most modern day video cards have very good algorithms which do a remarkable job of preserving details as they scale the texture down.
However, with larger textures, you can get more details in the texture, which will survive the filtering process better than a small texture would.
The visual difference, using a modern day video card, between 512 and 1024 is probably very difficult to detect from a distance. At 256, you might be able to tell, but it depends on the distance.
At 10,000 feet, I doubt there is anyway you could tell the difference between 256 and 1024 on the terrain. But in close distances during a dogfight, you probably can discern a difference.
We start with 1024 size textures/bitmaps, so there is a lot of detail that usually will be preserved as the texture is scaled down.
This is all pretty general. It is difficult to be exact as there are a high number of variables involved.
