Assuming you are using windows and have office: open the file and find the photo you want. right-click on the file name or thumbnail that you can see. Choose 'open with' from the long list of options, and in the next list that opens find 'microsoft office picture manager'.
In picture manager you will see a button at the top of the screen that says 'edit'. Click this and you then get on the right side a list of options including 'resize'.
The sizes are in pixels. As a guide,a computer screen is about 1000 pixels wide.When you have chosen a size and 'OK'd it, go to the top left of the screen and chose file > save as. Give this re-sized photo a new name, then click save. Then when you exit picture manager, choose 'don't save' when prompted, and in this way you will keep your original picture intact, but have a small version for use elsewhere.