What size? What vent system?

Time for a new gas water heater. 3 bath house but only 3 of us living there. Company only once a year. Furnace has a hot water coil but hot water circulates back to heater. Not sure if a 50gal, 40,000 btu is enough. Have a 30gal feeding a 50gal right now. Seems like overkill. Current model is a direct vent. Heater is in a 10ft tall crawl space. Do I have to use a direct vent system?

You really have 3 different situations;

1. Does the furnace have any control to prevent it from overheating the water heater stored water
2. Do you need 80 gallons of hot water
3. Why was the direct vent installed

All of these will affect your final decision some way. For the average family, a 50 gallon conventional water heater would be adequate.