I have one of the 45k btu heaters in my 24x24 building, R13 in walls, ceiling is R38 for first 6 feet on each side and R19 for the 12 feet in the middle (Has loft). it's more than big enough. If your insulation is the same or better than mine, I think one 45k would do it, but one 75k unit will heat it up from dead cold faster. For reference the furnace in my house is 65k btu input.