It depends on the species of horsefly. Some species are quite small under 1/2". The largest horsefly in North America is the Tabanus atratus or Giant Horsefly. They are often over 1.5" and it isn't uncommon to find them over 2".
http://images.whatsthatbug.com/imagesblack_horsefly_ruler.jpg
Has a picture of a Giant Horsefly that is slightly over 2".