Hard to say without knowing how high off the vanity the light fixture is. Tape it out on your wall and measure. If the measurements are not a standard size, get the next smaller standard one or have them custom made to fit. Most picture framing places will do custom mirrors with their frames.

Thank you! Would you use the same size (width) mirrors - that is what is hanging me up.

If the vanities are in different locations, then it doesn't matter if they are the same size.