Resizing photos is simple and there are hundreds of programs which can do it. One example is Irfanview which is a free image-viewing program with loads of image-manipulation options built into it. (It deserves a place on everyone's computer because it's vastly better than the image viewing & editing functions provided with Windows):
http://www.irfanview.com/
Open a picture in Irfanview. Go to Image > Resize/Resample. Set your desired size (taking care to select 'cm' rather than 'pixels', and click 'OK'). Go to File > Print and check that 'original size' is selected. Click on 'Printer setup' if you need to tell your printer that you're using photo paper. Click 'Print'.
(If your picture is the wrong shape, such as rectangular when you need it to be square then, before resizing as above, click-&-drag your mouse over the part of the picture you wish to retain and then go to Edit > Crop Selection).
Alternatively you can print photos to exact sizes from any program that can import them, such as Microsoft Word. See my instructions here:
http://www.theanswerbank.co.uk/Technology/Computers/Question918978.html
Chris