Sizing level shifter

Below is the schematic of a simple level shifter.

Sizing this circuit is not trivial. There is some feedback involved. I think MN1 and MN2 should be stronger than the PMOS transistors. Otherwise out does not become zero. For WP=2.5*WN, I used the following sizing for the transistors:

• MN1 : 5*WN
• MN2 : 5*WN
• MP1 : WP
• MP2 : 2*WP
• inverter : 3x standard size (NMOS=3*WN, PMOS=3*WP)