There are tons of different species of deer. Red deer which are actually small Eurasian elk are huge, but their size varies considerably, 200-400 pounds for bucks and about 3/5 that size for does. They say White tail deer in the states dress out 40% meat although I'm having trouble believing that. A big elk will weigh typically 800 pounds, will field dress to 540 pounds and will yield 270 pounds of meat. As what you will be eating is most likely red deer, say you get a 400 pound animal, 135 pounds of boneless meat should be a reasonable expectation. There are many other species of deer, some larger some smaller. Pier David's is 600 pounds and extinct in the wild. I would promote eating them because it will encourage the preservation of the species. Fallow deer are another common deer species in europe and white tails have been introduced.
