Firstly, crop your pictures into a square format by using the 'Manual' crop setting in Picasa:
http://etc.usf.edu/techease/win/images/how-do-i-crop-images-using-picasa
(The actual size doesn't matter at this stage. Just make them square).
Save your resultant image.
Then open a new, blank document in Microsoft Word (or any other word-processor or desktop publishing program). Go to Insert > Picture. Navigate to the location of your image file and double-click on it.
You'll now need to get the size right. In most versions of Word (and similar software) you can right-click on the picture and select 'Format Picture'. You'll then find an option to determine the exact size. (With some versions of Word, right-clicking produces a sizing box, at the top of the list of options, that can be used without needing to click on 'Format Picture').
If you want to print several pictures on the same page, hit Return a couple of times and then use Insert > Picture again.
When you've finished, print onto photo paper and reach for the scissors.
>>>>>>>>>ALTERNATIVELY>>>>>>>>>>>
Download & install Irfanview:
http://www.irfanview.com/
Crop the image to a square shape by holding your left mouse key down while you drag to create a box over the desired area. Hit Ctrl Y to complete the crop.
Go to File > Print and select the 'Custom' option, enabling you to choose 5cm as your preferred size.