.......... I've been lazy and only using iPhoto which doesn't seem to let me do that.
Hopefully it does and someone will instruct me.
I use Photoshop but here are the steps in iPhoto:
Select your photo in iPhoto
Notice in the lower left it gives you the size and dimensions. (red arrow)
On the top menu, select file then export. You will get the following pop-up window.
This is where it all happens.
Select file type (kind) Jpeg
Select Quality (this is what Matt was referring to)
Select the Dimension and give it a size -or- as I have done just set the largest side and it will scale accordingly.
Set the file name etc. in the drop down.
Here is the result in the above photo with the settings from the pop-up window shown.
Notice the file info in the lower left.
Hope this works for you.
Matt, Feel free to move this to the appropriate place.