Generally speaking, bathroom mirrors should span a good few inches less than the bathroom vanity unit or basin area they tend to be positioned above.
For instance, if you opt for a bathroom vanity with a width of 1200mm or 48 inches, you should typically select a mirror (frame inclusive) that doesn’t exceed 1200mm or 48 inches. This is to ensure that the mirror doesn’t become an overbearing feature in comparison with other bathroom furniture.
If you would like more advice regarding bathroom layout pertaining to bathroom vanities, take a look at our Buyer’s Guide To Bathroom Vanity Units.