I think using the same size mirror for both should work fine. How big are your two vanities?

49 long and 58 long.

Are the sinks the same size? Usually the mirror is sized with the sinks, not just the vanity. What about lights?

IMO we need pics since there is no mention even of style.

Vanity size is the only difference

Same mirror, same size will work.

Yep... like Suzanne said. Get same size mirror. You'll likely never notice the difference in vanity size, but you for sure would notice if the mirrors were differing sizes.

Thank you!

They can both be up to 75% of the width of the two double doors under the sink.

That is very helpful to know about the 75% width. The smaller size mirror will help distract from the wall size difference.