Simple Transistor Model
In inverter, only one transistor in path, so no load delay will be approximately Rds * Cd. (Cs does not affect delay because tied to Vdd or Gnd).
For inverter size N, Rds = Rk/N (Rk is Rds for size 1) For inverter size N, Cd = Ck * N (Ck is Cd for size 1) For gate size N, t0 = (Rk/N) * Ck * N = Rk * Ck