You can even do one better, and use MS Paint!! Simple and very very easy for those with lack of software.
All you have to do is go to the Image tab on the toolbar, and select stretch/skew (shortcut ctrl-w). Under the % for horizontal/vertical stretch, alter it according to how much too big your pic is. (i.e. if you estimate it to be 4 x too big, select 25 in each of the boxes so it quarters the size).
You can then check the size of your newly resized piccie by selecting attributes under the Image pull-down menu on the toolbar; this will show how big your little picture is in pixels.
It is fairly obvious once you get the hang of this on how to measure/crop images pretty accurately. My guesses were normally close to spot-on when I did it this way (we had a software drought. Now I use Microsoft Photo Editor which is part of the Microsoft Office Tools package. It helps if your computer is running correctly!)
Hope this helps!