it depends on how close the sink is to the end. I'd scale them as for individual vanities, like 30H by 24W, but you could go a little wider if space allows. Just keep each centered over its sink.

Traditional Bathroom- Bath Vanity · More Info

Southboine Residence · More Info

Thank you for the tip, it's a good idea to measure them separately! It hadn't occurred to me since it's one continuous counter. Thanks!

We have s 90 inch vanity with 9 foot ceilings. . Should we have two 40 x 28 foot mirrors or two 36 x 24 inch mirrors? We are remodeling so sink placement can change.