You can measure girth with a tape rule. One of those flimsy tape like rulers hehe. Place the end of the tape in the middle of your member then wrap the tape rule around until it meets up with your starting point. Then read what the tape rule says and thats your girth.
As for warming down, there are many different ways to do this. You could have a shower after each girth excercise, or use a warm cloth. If you look around the Newbie section there should be something about warming up and down.