1. Go to image in menu bar: go to image and resize from there to whatever size you are wanting your image to be. You can choose inches, pixels, or other options.
2. Go to file and save as use whatever file format you want.
I usually resize my image to a smaller size then Save as 01a, then I go back to the original image and undo the last command so it will still be the larger file size.
Example: Original Image: MagickMuffin01 Save as: MagickMuffin01. After resizing to smaller size from the image menu Save as MagickMuffin01a. Go back to the original and undo, so it will be the same size before altering it to the smaller size. I usually save my pics in PNG formats.
If you would like to see some of my Flickr pics,
http://www.flickr.com/photos/51567466@N00/?saved=1
Hope this helps.