One way for you to determine sizes is to get some graph paper and make a scale drawing of your vanity wall. That will help you select the sizes of your mirrors and lights. As to the design of the frames and sconces that will be more subjective. Pick your favorites and you will enjoy looking at them every day.

-Babka

What Babka says. Also think about if you want the sconces over the vanity or to either side. With a 48" vanity and a 40" mirror that leaves 4" either side of the vanity. No room to have the lights over the vanity.

Draw it up on graph paper.

Look on houzz and evaluate the mirror, vanity, and light placements. What do you like?

Also think about if you want the sconces over the vanity or to either side. With a 48" vanity and a 40" mirror that leaves 4" either side of the vanity. No room to have the lights over the vanity.

Which means the only possible combination is the 36" mirror with the Pottery Barn sconces.

My contractor actually penciled in the mirror and sconce placement right on the sheet rock. It really helped to have a visual.