I'm not exactly positive what area to measure but with calipers, the measurement of the stem where it enters the hole is 1.35" diameter and the handlebars where the stem attaches is 1.25" diameter.
...and yes, I put on the old man's seat cause my butt was killin me! i also added the handlebar extenders which didn't do the trick. I need to raise the seat about 4 inches but if I do my riding position is way too far forward.
Here's some pictures: