When painting a texture, many times people will paint at two times the resolution, then size it down. Meaning if the final map is a 512x512, they will paint at 1024x1024. I recommend staying at true resolution whenever possible.
If you are just starting out at texturing, painting at the actual resolution is a good habit to get into. There are many reasons for this. First and foremost, is the time factor. You do not have to worry about any details that cannot fit into the end result map, because it IS the end result map. If the pixels hold it, it will show up in game (if the camera/player gets close enough) Secondly is how crisp your end result will be. I have said before, and Ill say again, you as the artist, want the control. any uniform application by a computer will end up looking piss poor compared to the same sort of effect applied by a skilled artist. Each pixel in the resized map is the result of the average of four pixels at the larger size. You can place single pixel highlights, as well as razor sharp single pixel seams and shadows. They will not be blurred to obscurity by Photoshop's algorithms.
Common reasons people argue for painting at double res:
But I can be messier/faster/scribblier and then size it down and it looks "right". This is not true, it only makes your mistakes less apparent. Their is a reason that art teachers tell you to "draw big" It is because it makes your mistakes apparent. If you cannot make it look right at the small resolution, you are not making it look right when you resize either. It is just that your mistakes become less apparent.
But I can get subpixel detail! Again, not true, there is no such thing. A pixel is a pixel is a pixel. You can have detail that is implied as smaller than a pixel, but it is just as reproducible at resolution, as it is double size, you just have to know how to do it. and it will read better because you did it at true resolution, than if you resized down. One particular thing that people like resizing for is hair. A good way to learn how to do implied sub pixel detail, is to actually resize something, and look to see what the pixels are doing that implies that thinner than a pixel detail. Once you learn these "rules" you can use them or adapt them for your needs, and be faster because you are painting them at resolution.
Again, if you are already fantastic at texturing, and you resize. By all means, continue doing it. It is, and always will be, the in-game results that count.