D Cells, (or the British equivalent) C Cells would look pretty tiny hanging off of Mr. Prowse's backside. ;)
As an aside, I'm surprised they still make C cells. There don't seem to be many gadgets or toys that use them today. (AA's or smaller are dominant.) I have an old handheld electronic game from the 80's that requires two C's plus a nine volt!
This also raises the issue of where Threepio's batteries were hidden for wide shots where his eyes are lit up. That panel on his back?