In iPhoto, select your photos and from the File menu choose Export. On the File Export tab, under Size, set the width and height under "Scale images no larger than" and then click Export. Choose a folder for your resized exported photos and you're golden.
Second, for non-iPhoto users unafraid of AppleScript (and there's nothing to fear), this excellent Mac OS X hint provides a script you can save right to your dock or Desktop. To resize your photos, drag 'em from Finder and drop 'em onto the script you set up and - shazam! - you've got resized copies of your images named scaled.Original_File-Name in the same folder as the originals. Hat tip to Stompy for that gem.