Some of the newer text file formats for art assets (.lin files, roads, etc.) require you to specify how wide the texture is.
Huh? Why does X-Plane need me to tell it how wide a texture is? Doesn't it know?
Well, first, the answer is: no! When X-Plane builds the roads and draped lines, the texture that is needed may not actually be loaded. It may only be on the todo list of textures that need loading, and if you have orthophoto scenery, that list could be very, very long. There is no guarantee that the texture is loaded before the geometry gets built. If you have a multicore machine, both might happen at the same time!
So the first reason why the road and .lin files require you to declare the size of your texture is: to allow X-Plane to build the mesh on a separate core from the one that loads the texture, for faster loading. Isn't multi-core fun?
There's another reason, and it's a little bit more subtle: you can change the size of your X-Plane textures. Imagine that you get a new graphics card and you decide to "upgrade" your scenery pack. So you double the size of all of your textures, and then go in to photoshop and add more detail.
But if X-Plane were to look at the size of the texture, then all of your .lin files will be wrong! For example, let's say you have a .lin file with a line from pixels 0 to 16 across the texture. When you double the size of your texture, you'd have to go in and change this to be 0 to 32 pixels across the texture.
But here's the trick: since X-Plane gets the size of the texture from you and not from the actual texture, you can just leave your .lin file the way it was - with the old texture size and the old coordinates. X-Plane will do the math out and generate correct texture mappings even though the texture size is double.
In other words, the important thing about texture coordinates is that they are self-consistent, not that they reflect the actual texture size. This is because internally all texture mapping is done in ratios.
(So why not just use ratios in the file format? Try entering the size of 10 or 12 lines as ratios and tell me whether you like it. The ability to enter the line coordinates in pixels is to make it easier to set up the line files from the image file.)