I have a giant mirror and I love it! I got it for pretty cheap at Home Depot. It is probably 3' high and 1 1/2' across. Since my 'workout' area is our house's breezeway, there are windows everywhere and no true places to hang a mirror. I prop it against the wall, and place my lighter dumbbells (3#s) infront of it, and in 5 years it has never fallen.
I can't see my upper body in the mirror when I'm standing, so I try to do most of my upper body work seated so I can check my form. For lunges and squats though, I can check my form pretty well.
BUT - what I really love using my mirror for is to check myself during Imax, MIC, Leaner Legs, and Boot Camp. I know that if I am really pushing it, my knees go just outside the top of the mirror's view during high jogs and plyo heel digs. Also, when I do airborn jacks or airborn jumps, and when I am wearing bike shorts (as opposed to leggings) my shorts disappear just over the top of the mirror (that sounded bad, didn't it) - so all you see is leg. I know that if I can still see the bottom of my shorts during an airborn segment, I am not jumping as high as I should be. And - when I do 'low ends' I know that on a low end, the top of my head should not go over the top of the mirror so during a low end if all I can see is my nose or chin, I know that I am not squating high enough. I love my mirror. And I'll be honest with you, it's nice to be working your delts or bis and catch some definition in your reflection.
Holy crap, apparently I needed to write a book to answer your question. Sorry. I just finished Annette's 'oh my mish mosh' and I am a little high on endorphins!