Editing the image size can be done when editing a page and inserting the image. Click on the image after inserting it to see an "Image Options" popup. Click on the popup to find some editing options, including sizes. There are some default sizes, but you can also define either Height or Width to whatever you want. It does maintain the ratio so edit only one and it will fix both. This only changes how the image displays. It does not change the original image size. EDIT: See mzimmerman's answer below for more on this.

You can also drag the corner of the image to resize, but Image Options can be easier to define a specific size, along with some additional options.