I think you're either looking for square inches (surface area) or weight, which would be based on the shape of the wood and the surface area of it. Chips are more surface area than cubes, so you get more flavor per ounce. But I've heard it's a more superficial character since it's mostly toast without any deep wood penetration (heh). Cubes need longer aging, which may not be desired for a beer.
The NB site sez about a 4 ounce bag of chips: "Used in the Whiskey Barrel Stout kit. Very pleased with the deep, rich flavor. While above directions use it in 25 gallons of wine, the kit uses the packet in 5 gallons of beer. " Now, that sounds like a lot of oak to me, but it depends what you're going for.
From the mathematical standpoint, you can approximate a 53 gallon bourbon barrel as a cylinder of ID 22" and inside height 32". The surface area of this is pi*22*32 + 2*(pi*11^2), which is almost 3000 square inches, or about 56 square inches per gallon.
Now, if a cube is on average 1" on a side (totally guessing here), you have 6 square inches of surface area per cube. The density of american white oak, according to the internet, is 47 lb/ft^3, or 0.44 oz/in^3. So you need ~ 9 cubes/ gallon, or 20 oz of cubes per 5 gallons. Holy s$#%! I guess the moral of the story is, this is why bourbon is so damn oaky. Unless you want to replicate the flavor or a bourbon, do not use this much newly toasted oak in your beer.
This totally useless post has been brought to you by too much coffee.
Edit: I guess you could boil/pre-soak the cubes in vodka to mimick used barrels and throw them in for extended aging. This would be an interesting experiment as opposed to using fewer new chips for a short period of time.