The largest "verified" field dressed whitetail I ever heard of was a 355 pound war horse killed by a guy named Horace Hnikley in Maine back in 1955.
http://www.sportingjournal.com/main9.shtml
Largest field dressed whitetail I ever saw weighed in Maine was about 225 . The largest I ever saw actually weighed in Maryland was also over 200 . The largest I've personally seen weighed in Virginia was 185 dressed . Now in PA , VA and MD of all the bucks I've seen or killed etc were more in the 125-140 field dressed class .