There are no BMP specifically responsible for a single vehicle or a single class of objects, the textures are packed and arranged by material type, which is of course the best possible way to save performance, because it's much faster to have few large textures than many smaller ones, especially if they are optimized according to material properties, which they already are.
Believe me, the scenery is already as optimized as it can be...
Also, it would't make any sense to try to "optimize" the texture yourself like, for example, by resizing them.
The "Global Max Texture Size" slider does exactly the same. Full slider to the right, and the maximum texture size allowed will be 1024x1024. One notch to the left it's 512x512 and so on. So, if you simply act on the slider, you'll have exactly the same effect than manually resizing the bitmaps, with the added convenience that the setting can be adjusted without even leaving flight sim.