10x8 should be fine... and in terms of megapixels - well technically 6-8 would be fine (at 4:3 framing btw)... work at a 300ppi and a 10x8 = 3000x2400 - just over 7megapixels...  Problem comes with quality of shot (meaning glass in front of sensor) which basically means i would be happy taking that photo with any entry level DSLR out there... (and maybe some compacts would do a decent enough job). Good lighting will help too - bouncing a flash around is a good idea if you can or get them outside if it is a sunny day and shoot them in a shaded area against a plain wall or something so the sky lights them.

But you mentioned a 2:1 aspect ratio which is probably a better idea...and just have head and shoulders, maybe torso and 3 rows of ~10. Thing is printing weird sizes is more expensive as i seem to recall and you mentioned a "prop"... could be a sticking point...

"In every colour, there's the light.
In every stone sleeps a crystal.
Remember the Shaman, when he used to say:
Man is the dream of the Dolphin"

Rich Meadows Photography