I've just looked at a few resistors of various sorts and come up with a rough heuristic (a guess in the form of an equation) for the probable power rating of a resistor:
P = L^2 L = length in cm, P = power in watts.
Admittedly the larger resistors I looked at need to bolt onto a heatsink, but it sort of works as a sanity-check - the area of a resistor is roughly proportional to the length squared, and heat has to leave through surface area. It even works for an electric fire where a 1kW element is usually about 30cm long (but runs rather hot as its too skinny for its length!)