I was looking for the same info just this morning, and found this from a previous post.
"
have a macbook also. I find the picture I want to resize. Under the file menu I click export, then click file export tab, JPEG quality drop down menu, click medium, size drop down menu, click medium, then I click the blue export button at the bottom right..I then choose export to desktop. Once in YT, then I upload the pic from the desktop rather than iPhoto since its obviously smaller."
Worked like a charm, and easy peasy
. I actually tried the minimizing program mentioned first, but couldn't get it to download on my mac.