Depends on the software you're using, but if you're using Microsoft Paint, here's how:
Start > Programs > Accessories > Paint
File > Open
Select the photo you want to resize
Image > Stretch/Skew
Type the percentage you want to stretch it. 100%=no difference, 50%=half size, 200%=double size. Type the same number in Horizontal and Vertical if you don't want the image distorted.
If you want to resize it to a particular pixel size, go to Image > Attributes which will tell you the current pixel size. Work out the percentage with (desired size / current size) * 100.
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