There are a # of online calculators but cubic feet is the # you need (not sq ft) as well as type of construction. Your buddy will know and should be able to calculate your needs based on avg temp and the temp rise you want. Anyhow, mine is the same as yours except I only have 10' ceiling. At 13', you should look into and consider radiant heat. Cheaper operating costs and "instant" recovery from door openings.
