Resizing Mirrors- If you want to do this yourself it is pretty simple with few inexpensive tools. you will need 1. a metal straight edge (ruled) at least 18" or bigger. 2. a diamond wheel glass cutter available at craft stores that sell stained glass products.
3. 3 in-one-oil or olive oil works just as well to lubricate the glass.
The trick is to cut from the back thru the 'silver', use the straight edge as your guide, make sure the wheel is perpendicular to the glass, the cut must be continuous, don't start and stop, oil the wheel before you start. When the cut is finished, turn the glass (mirror) over and lightly tap the line and it should break off cleanly, practice on a piece of picture frame glass or any scrap glass you can find before you start on the real thing, good luck!
P.S.
take it to a glass shop if you have curved lines, they are tough to
cut without special templates.

Leslie9220 Have fun !