10 inch. single cut mill file, to carborundum stone, to series of Arkansas stones.
If the axe is already sharp with a good profile ... then just to a series of Arkansas stones.
 
http://www.fhwa.dot.gov/environment/recreational_trails/publications/fs_publicat ions/99232823/page15.cfm
 
The whole axe manual here:
http://www.fhwa.dot.gov/environment/recreational_trails/publications/fs_publicat ions/99232823/index.cfm

A tomahawk will probably be used more for "camp chores" than for heavy wood processing jobs, so I`d say use a file like Steven suggest, and file an 1/2" edge on it and hone it pretty sharp (I got one or two axes with hair flinging edges but that`s not needed at all for most jobs ).  Use it for your purposes, if it don`t hold out just round it off (convex it) as needed.  Each axe/user/use require an individual edge for best result.
While I have seen several Cold Steel tomahawks I don`t own one and I don`t remember what kind of factory edge they come with.  I notice that most axes I see in the stores these days got much thicker factory edges than back in my days.
 
timann
 
PS We keep a small hair flinging sharp axe by our "clan`s" electric wood splitter for when wood fibers or bark hold the split wood together, makes for a fast fix when in a hurry.  I simply hone it when it dulls.
 

I use a file. I have never tried to get a razor edge on an axe, but it seems an arkansas stone will do the trick for you.

You should get "The Razor Edge Book of Sharpening."
 
I have a copy. It'll teach you what you need to know. This guy SHAVES with axes that he has sharpened. He knows his stuff.