There are several ways to re-size images. By far the simplest way -- and the method we recommend -- is to re-size the image before loading it onto your website. There are a number of graphics packages you can use, ranging from the complex and expensive (Adobe Photoshop) to the free and easy (picnik.com). We recommend that novices try the services here: picnik.com. Images can also be resized in MS Paint using the "Stretch/Skew" feature. Once you have re-sized the image to the appropriate size, simply load it via "Content Upload" so that it can be available for use in any page.
If you have already loaded your image and/or you just don't want to resize and reload, then here are instructions for resizing how an image displays: https://blueutopia.zendesk.com/entries/108742706-How-do-I-change-the-size-of-an-image-in-a-page-