If you have some type of photo editing software installed on your puter, open the photo you want to size and you'll find the sizing tool in the tools tab probably. If you dont have any photo editing software, here is a good free one to download.
there is a free download to a software all you have to tell it is the physical size you want the final product to be and it'll shrink it down what I really like about it is the ability to tell it to resize an entire directory of pictures, and when you tell it to do so you can make a copy or resize the original.