It depends on the configuration of your 42" vanity.

Are the sinks centered in the cabinet or are they off to the right or left??

I hope you didn't purchase vanities with feet.

What are the vanities? The sink is centered on each? You are butting them together? How high is the ceiling? Do the vanities sit together in a niche? Or are they against one wall and the other is open. Or there is no wall at either side? It's a new build? It's a remodel?

You have to provide a drawing, pictures of what you purchased, show the plan for the bath, whatever it is. These are vanities bought online, or being built?

So unless I simply say get two mirrors, 36 inches wide and 48 inches tall, or get two round mirrors at a 36 inch diameter? It's all just a big guess, isn't it??: ) Yup........

https://www.houzz.com/discussions/6411000/how-to-houzz-how-to-ask-questions-how-to-get-helpful-answers-and-e#28784466

Ditto.