Logical Effort Inverter Sizing
It's hard to calculate the total effective capacitance from model parameters because the capacitance from the gate to body of each transistor will change as \$V_{GS}\$ changes and the inversion layer forms.

My approach has always been to drive the inverter input with a constant current (in a simulation of course) and see how long it takes the input voltage to rise from ground to the supply voltage. Then calculate

$$ C = i \times \frac{dt}{dV}$$

where \$i\$ is the constant current, \$dt\$ is the time for the input voltage change, and \$dV\$ is the magnitude of the voltage change (usually just equal to \$V_{DD}\$).