Any system camera will do. The smallest one as far as i know is the Samsung NX mini, but the sony ones (NEX-3 or Alpha 5000) are pretty small as well, Nikon and Olympus have some nice ones too.
I treated myself to a sony A7, which is not the smallest one around but the smallest one with a full frame sensor. Downside is that the lenses need to be bigger, upside is the great picture quality especially in low light situations. For most situations during a tour the kit lens (28-70mm) is actually not bad, although the whole system is smaller with my 35mm lens. That isn't really suited for airborne landscape pictures though and certainly not suited for air to air pictures, for those a tele lens would be better, its great for street life pictures and groundbased landscape pictures though.