Yes, the physical size is the only way to determing the power rating of a resistor. Although there is no set standard, most manufacturers pretty well stay close to each other in this regard. The "standard" sizes are for 1/16-watt through 2-watts only and then for carbon composition or carbon film only. Beyond 2 watts or if playing with metal film, wirewound or metal oxide, all "standard" bets are off!

I might add that although a 1/4-watt carbon film and 1/4-watt carbon composition resistor are about the same size and have the same power rating, they are NOT really the same in power rating. For continuous power dissipation, they'll both handle 1/4 watt. But for short bursts of power over the rating -- that is, pulsed power that may average out to 1/4 watt -- the composition resistor will handle it while the film resistor will smoke on you. The composition resistor has a lot more physical bulk in the resistance element and can absorb power surges while the film resistor is nothing more than a "skin" of carbon deposited on the surface of a ceramic slug and will flash with a surge of power.

Dean

__________________
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.