What's the right size pipe to buy for gas supply for this furnace?
Generally, the bigger the better. Assuming you already have gas, you should match what you have already installed. The size shown is the minimum for each of the 2 gases, Propane vs Natural Gas. You can use a pipe nipple/reducer to reduce the size of the valve inlet as needed. As for the other numbers, BTU is the heating factor (British Thermal Unit), higher number means it will heat a larger space, or a smaller one faster. The dimensions are used for the place you plan to install the furnace. Your gas valve is 1/2"ID(Inside Diameter), your measurement is 3/4"OD(Outside Diameter). So a 1/2" ID NPT(National Pipe Thread) is what you need at the valve, however, up to the valve you can plumb 3/4"ID. If you are doing a new install from the meter, best to go with the 3/4" in case you plan to upgrade in the future or add additional gas appliances.