Summarize this article:

You should be able to see yourself all at once in any mirror you use in your home. This doesn't necessarily mean you need to have a home full of full-length mirrors, but you should avoid tiny mirrors, or mirrors made up of individual tiles. You should be able to see your head and shoulders in the mirror at the same time. You should avoid selecting mirrors with distorted surfaces. This also applies to mirrors with antiquated surfaces. It's believed that looking into such mirrors will make you feel distorted yourself, and this is bad for your home's feng shui. Try to avoid using round or oval shaped mirrors in your home. Square or rectangular mirrors are generally thought to be best for creating good feng shui. You shouldn't hang any mirrors in your home that are broken or cracked. If a mirror that's already in your house breaks, take it down immediately.

Summary:
Use whole mirrors. Don't use mirrors with distorted surfaces. Use square or rectangular mirrors. Avoid broken mirrors.