There are dozens of different ways of doing what you require. There are plenty of image-resizing programs freely available or you could use a DTP program. However, for such a simple (and possibly one-off) job, it's hardly worth the bother of downloading and installing new software.
Simply use Word (which I assume that, like the majority of people) you've already got. Open a blank document. Go to Insert > Picture > From file. Navigate to the location of the relevant picture and double-click on it. This will import the picture into your document. Then do either of the following:
1. Left-click on the picture. Then use the 'handles', at the edge of the picture, to drag it to the required size. (Use the handles at the corner of the picture to maintain the correct aspect ratio. Using the handles at the top, bottom or sides will make the picture fatter or thinner). This method is probably good enough, as long as you don't mind a millimetre or two error. For a more exact method, use . . .
2. Right-click on the picture. Select 'format picture' and then the 'size' tab. Enter the required dimensions and click 'OK'.
Then simply print your Word document onto photo paper.
If you require the photographs for a passport application, you can check the required standards here:
http://www.ips.gov.uk/passport/downloads/photo s.pdf
Chris