If, I were to install a Hot Water Heater to be used in the Summer and shutoff my Oil Burning Furnace, what would be the most economical way to go? Electric, Propane or stay with my existing system. Would I install 40 gallon Tank or go larger? Presently, only one person using and would shut off in the winter. Short term, I know it would not be economical, how about long term?

Something you may not have considered is that since this is going to be a summer home and you may get tax credits to have it installed you might think solar. In your case it may make sense but do your research to find out.

Solar Water Heaters | Department of Energy

Solar Water Heater : Solar Hot Water Heater & Solar Thermal Systems in USA

A lot is going to depend on electric & propane costs in your area.
In my area, you can buy propane much cheaper if you own your tank. If the propane company owns the tank, they charge much more per gallon of gas.

40 gallons should be fine for one person. Natural gas would be cheapest if its available in this home. Propane is next cheapest but has less BTU per cubic foot and electric is most expense in most areas but you'd have to price it yourself.