Every engine from Quake I to Crysis uses downscaled textures. If you load a 1024x1024 texture, it compute 512x512, 256x256, ..., 1x1 textures called mipmaps. Then when an object is rendered it picks the texture where the resolution best matches the screen resolution. I.e. if your 1024x1024 texture appears on a 4x4 quad on the screen, it will automatically use the 4x4 texture.
This means that you can see the super-high-res textures only when you're sufficiently close. You can use the switch /r_colorMipLevels 1 in Tremulous to see the effect. All downscaled textures will be coloured differently, only the base texture keeps the original colour.
Old engines may have problems with texture sizes that are not powers of two and rescale them, which blurs the textures a bit, but most textures have power of two sizes, so that's not a big problem.
If you only want to add some noise to the textures to make them less boring at close views, you can use detail textures, i.e. you use the mid-res textures that you have now and add a noise texture in a second renderer pass. The noise texture can be repeated over the surface, so it does not need to be big for high resolution.