Texture Ratio

What should be a good texture ratio for a mobile game object
240x240?
or 1040x1040

 

It depends entirely on the object being textured. How much of the screen will the object take up? How many other objects are in the scene that also need a share of the memory? This isn't really something that is going to have a concrete answer. It's all part of the art/science voodoo dance that is game development and is going to be project and scene specific.

 

Its a car

 

You don't think about such things so early, just make the gfx as big as you can at start. Then finish the game and see how much memory you will even have available for each thing in your game.

 

Unity doesn't really care about your source image size. You could make every source texture in the game 4096x4096 if you wanted and then change the import settings in Unity to scale it smaller on import. You can start at 1024x1024 and scale it down until the quality difference is noticeably worse. Then, if you want, you can re-author the image at that resolution if you are worried that scaling artifacts are affecting the look. You may need to scale it down further if you have a bunch of cars on screen. It just depends on what the resolution is before it looks bad and the memory requirements of the rest of the scene.

 

Just to make sure you know it, the resolution is usually a power of 2, so in your case it should be 256x256 and 1024x1024.
As for the question - It's all up to you.
As HemiMG said, the good thing about Unity is that it allows you to easily downscale the textures in the import settings and instantly check how it looks in game. So, you could create the first car with a 1024x1024 texture or even 2048x2048, import it into Unity, and start downscaling it, until you find a good compromise between size and quality. From then on you could use that smaller size to create all the other similar textures, so you don't spend too much time creating the detailed texture that will end up downscaled.

 

But unity's downscale is pretty bad in quality compared to the one in photoshop.

 

Dats the problem

 

I'm pretty sure That both I and devil_inside offered solutions to that problem.