Determining the 1X sized gate (one-stage gates)
Start with an ntree/ptree that has the same W/L equivalent ratio of a 1X inverter. If 1x inverter is W/L Nmos = 0.9U/0.6U, W/L pmos = 1.8U/0.6U, then:
To pullup, only one PMOS needs to be on, so equivalent W/L is just one of the pmos transistors. To pulldown, both NMOS must be on, so ‘L’s of the two transistors add. These trees will have equivalent delays to a 1X inverter, but DC switching point will be closer to GND.