Repeat after me: 1-1/2″ for kitchen sinks, 1-1/4″ for bathroom sinks. Or, you could construct an adapter like this….

Follow along with me from left to right:   we start with a 1-1/2″ to 1-1/4″ drain and trap connector, which is required to connect to the steel drain pipe. But then things go downhill, as we have a 1-1/4″ to 1-1/2″ adapter, a 1-1/2″ (kitchen sink) P-Trap, followed by a 1 1/2″ to 1 1/4″ adapter to connect to the bathroom sink drain.

I’d like to think that somebody decided they really wanted a larger P-Trap on the master bath sink…but I suspect that the plumber only had a kitchen sink P-Trap in his truck and didn’t want to make a special trip

Since I needed to change the length I went ahead and used 1-1/4″ throughout. I think it looks a bit nicer. (I also sealed the hole into the wall with expanding foam while I was under the sink.)