The size of the mirror on my Silva Ranger works well for this. I'm not sure why any quality mirrored sighting compass would be inadequate actually.
The aiming method I use:
Hold mirror in right hand, near your head. Extend left arm out and make an L shape w/ thumb and index finger, to "box in" your intended target (get the target to appear, say, 1" from the web of your palm). In most conditions its easy to see the mirror's light against the back of your hand, angling the mirror side to side so that it flashes light onto your target. Of course if your light source is a headlamp rather than the sun, then you'd have to strap the lamp to a trekking pole, or have a friend.
I should state I've never signaled aircraft this way, but its been visible to members of my party from maybe half a mile away. Realistically, in most parts of the US at least, I wonder if one could count on an aircraft crew member responding in ANY way to only a flashing light on the ground. Expecially since one has to stand still to aim the mirror. A pattern of serious signal fires, and a guy lying down making frantic snow-angels in the dirt, would probably send a clearer distress message to low flying planes, etc.