If you drain a shower with a 2" trap and a 2" horizontal pipe going to a 1.5" vertical pipe down one story, will the drain work properly?

You've described what's typical.

It will work as long as the vertical one is vented.

Thank you, Rick. That is music to my ear. I was very worried about passing inspection. Then why is there is 2" requirement since it eventually drop down to 1.5"?

Debbie

Pass inspection?
With who?

That there is an inspection involve might change the situation - depending on who is inspecting.

I figured your use of 2" was your own preference... and that the general layout of the drain was the key part.

Can you give some more detail about who is inspecting and why?

I don't know if we will need an inspection by the local city of not; I was just writing off the top of my head.

This is the type of repair you can do without notifying the city.

Even if you had a contractor do it I'll be a dollar to a donut that he would not contact the city for a permit or inspection.