Do you have iPhoto? If so, you can export a picture from iPhoto using either the File menu (iPhoto 6.0) or the Share menu (earlier versions of iPhoto). From here, you get prompted with what size you want the picture to be in terms of pixels.
Using the Save As command from the File menu in Preview, you can change the quality of a picture (depending on which format you save to) however I'm not sure that you can resize them. 🙂