If that is the only thing you are going to run is a tankless water heater then I would suggest a minimum of a 100 lb tank. Should last you quite a while. best to talk to the dealer from whom you are going to purchase the tankless water heater from.
What you should do is figure an average run time per day that you will be using hot water. If you have 4- 20 minute showers, cooking, laundry, and dishes. Might want to figure in about 2 hrs a day. Its better to overestimate. Your tankless heater will have a btuh input. Its usually 120,000 to 160,000 btu.
Propane delivers 22,000 btu's per pound, or 91,000 btu per gallon. If you figure in 2 hrs per day, at 30 days you are at 60 hrs. Which using the 120,000 heater is 7,200,000 btu's. That's 327 pounds or 79 gallons.
These usage numbers are hypothetical and may or may not represent your lifestyle or usage. Figure out your usage and how often you want to fill the tank. Propane is running around $2.85 a gallon right now, so consider your usage before jumping into this. Your local propane supplier will have certain size tanks that they use, and have a minimum delivery charge/order.
Some areas will have minimum distances between the tank and the house.
I looked at doing this myself. After running the numbers, all I was doing was breaking even. If I didn't have kids, it would be a no-brainer.