It depends on what graphics program your using. If your using Paint Shop Pro it's "image->resize" on the menu; if your using Corel PhotoPaint it's "image->resample" on the menu; if your using MSPaint (my sympathies) it's "image->stretch/skew". Set your sizes as required. I'm not sure of the photoshop menu path to set size but it will be something along the same lines as one of the above.
Cheers,