I'd like to start by saying the Preview resizing, suggested above, is by far the best and simplest way to handle the task, however seeing that it cause some confusion, would something like that work for you: on secondary click you select Resize Image:
Then set the desired size (long side in pixel):
and get a reduced size copy of the original image, with 'resized' added to file name?:
If yes, PM me, and I'll email you an automator workflow file that will do just that.
Now, this is something I would never, normally, recommend to anyone, in fact, would strongly advise against that ... I was going to suggest you create your own Automator workflow, but perhaps, it might be a bit too involved