Adding on to what opt2not said, for best results scale it by a whole number. Nearest neighbor will keep your colors intact but proportions can get weird.
For example, if you take one pixel, which is a 1x1 square, and you want to make it bigger you can make it a 2x2 square, which will be four pixel, or a 3x3 square, which will be nine pixels, and so on, but you can't make it a 1.5x1.5 square. So if you scale by 200%, 300%, etc. every pixel will be perfectly reproduced whereas if you scale it by say 275% you will get wonkiness as new pixels have to be generated, altering the fidelity of the artwork.