A mirrorlike that would be similar in size to the inside top of the so-called "5 in 1 Survival Whistle." See: http://beprepared.com/product.asp_Q_pn_E_CM%20W400_A_name_E_[[ProductNameURL]]

I've got a couple, which I have in kits, mostly because I like the whistle with the hollow inside for inserting some makings for fire. The part of this whistle I like least is the so-called signalling mirror. It's not a mirror, but seems to be a piece of shiny silver cardboard like one might find in a frozen food package.

A real mirror as you propose is likely to be a lot more effective should the need to signal arise. A larger mirror is probably going to be more visible, it's going to reflet more light. But the smaller one should be better than nothing.