I know that whenever you have series transistors multiply the equivalent W/L of the inverter by the number of series transistors. In the parallel case W/L remains the same. I don't know how to apply those two rules to this problem. All of the examples I can find are far simpler than this and more symmetric.
How do I apply parallel and series rules to transistors?
If you have already sketched out the transistor diagrams take the worst case N path (4 transistors if I am not mistaken) and size these to give you the required overall w/l. The other transistors on the N side can be smaller, you have a set of three one of which is already sized from the above. Size the other two to match the needed overall W/L. Finally you have one which acts on its own size that one accordingly.
Repeat the exercise for the P side transistors and you are done.
