Well if your only wanting a heater without going through all the trouble og using the original stuff why not just buy a under dash drop in unit by one of the many companies that make them?
As for your question the side of the whole needed will depend on the size of the heater core that exits that whole. You will also have to cut out that round whole to the left of the rectangle whole (far left of firewall looking at your picture) that is where the blower motor is installed. That is of course if you want to use OEM heater equipment. Smoothing the firewall and using a drop in under dash unit is possible as well. You can get just a heater for $160 or you could get heater/AC/defrost or any combo therein from many different companies on the net.