The size of the texture itself has zero to do with the resolution.
You can make hi-res mods using the default texture size.
Blowing up the textures x400 is the standard set by old school modders, but by no means is it the rigid standard for texture size, and as I said before, the size of your textures has nothing to do with the resolution of the image, it's the resolution that matters.
For instance, take a default texture, blow it up, and does that make it high res? Nope, it still has the exact same resolution as before, but since it is enlarged, it becomes blurry and shitty.
The definition: "used to describe something such as a screen or photograph that shows an image extremely clearly"
So it can really be any size, but it has to be seen clearly.