First off, you do NOT want to use a "tee" to connect the 2 heating appliance chimneys. That is a no-no. One option is to use your existing chimney for the coal heater and use a power vent for the oil burner. The power vent would only operate when the oil burner runs. 2nd option is to add another chimney.
The hot air from the coal heater gets hooked into the cold air return of the oil burner. Then you can use the oil burner's fan to circulate the air. A good diagram on the ductwork install can be found here : http://www.leisurelinestoves.com/1904137.html
The outside cold air make up is connected to the combustion fan inlet of the coal stove.
As far as the sizing of the stove, what is the btu rating of your current oil furnace ??? Use that as a guide to determine what size coal stove to purchase.