For wall mirrors, the general rule of thumb when hanging them up is to install them at roughly eye level (I don’t know if you’ve ever walked past a mirror that’s too low or high for your height, but it’s a slightly disconcerting experience!).
The exception here of course is if you’re creating a gallery of wall mirrors, which is a great idea for a feature wall as it can really open up a space. It’s a good idea to use different styles of mirror here, e.g. modern, classic, and statement styles, but try to stick to the same color scheme so it doesn’t look too haphazard.
Floor mirrors are slightly more simple – great styling ideas include placing a large plant or floor pillow nearby for a contrast in shape. If leaning your mirror against a wall, try to create some friction by placing a grippy rug underneath or even better, adding it to a room with a carpet. With extra large or heavy leaning mirrors it’s always best to add some security in the form of wall hooks (longer styles won’t take away from the leaning style).