How big can flies be?
There's a pic of a 1.8 cm tabanid at the link, below. Tabanids include horseflies which are the heaviest of the flies.
If you are looking for a fly that covers a lot of real estate, the Tipulids or crane flies have legs that reach a maximum span in tropical species of upwards to 10 cm. They are very spindly and would not approach the weight of a big tabanid.