Viewing the source is the best way to find out although I beleive its font-size:10px; font-family:verdana,arial,sans-serif; line-height:13px; (I just matched some up yesterday although can't remember 100%)
I think for the size it depends how many ads display in the rectangle ad box. It seems to me that google sticks with the Arial font when it comes to their ads. There are just variable size differences. You may have to expirement with the size of the font to match it up with the ad.