meh, I don't personally like photoshop for spriting, then again, I'm just not that good with it >_<
anyways, for paint, first right click the picture and select "select all", then choose copy/cut.
Now, stretch the picture border to how large you think you might want it to be.
Now, repaste your sprite, and while the sprite is still selected, right click and choose "Stretch/Skew"
Now just choose how much you think it'll be.
The reason you should use the right click instead of going to the image tap at the top is becuase using the image tab, the sprite will get distorted, which using right click, no new colors will appear.