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.