Hi Parky, take a look at my site (
http://homepage.ntlworld.com/b.s.scott/index.html) for details of the 48" minesweeper that I built for steam power.
Obviously, in power/weight ratio, steam will never match ic or electric and since power required is linked to speed as well as weight it would depend on the speed you want your model to run at and its weight (displacement).
I'm no good at working these things out on paper, I just took a chance with the cheapest engine I could find and, luckily, it works well if enough for me.
I hope this is of some help to you.
Scottie.