Which version of the Game?
If you're talking about Stock SH3 then yes there nearly 2 sizes of every texture in the game. This allows the options for High & Low Quality Textures in the main menu to work.
As for resizing. I wish it was that easy.
Resizing a 512 x 512 texture to 1024 x 1024, will actually give in a poorer quality result. A graphics suite, during resize/upscale will need to add pixel information, based on it's neighbors colour, this usually results in a blurry final image. It can't just make that stuff up.
The only way to successfully improve an existing 512 x 512 image is to use the upscale as a base in a gfx suite and then design/overlay your new creation which by default will be at the higher res.
Here's an exmaple. Sorry in advance, I just googled something.
256 x 256 Icon:
Upscaled to 512 x 512:
You try to gain quality but you actually loose it.