Finding the wattage rating of resistors and variable resistors?

Is there a way to find the wattage rating of resistors and such mathematically? or is it something that has to be tested in real life?
I have quite a few resistors that I'd like to use, but don't know the wattage rating of them. I only know their resistance, and I could calculate other values if I put voltage across them with a multimeter. 
Also, what type of variable resistor would you recommend to control a 12 VDC device that draws 12 amps?

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A similar question, about the power rating of resistors, was asked unto this forum before, and this page:
has my attempts to answer this question.

Regarding the math, the power dissipated by a resistor flows out of its surface as heat.  So the amount of power a resistor can dissipate (without getting so hot it burns itself up) tends to increase with its surface area, which increases with its size. Basically what Iceng said.

A variable resistor to put in series with a 12 VDC source, and a load that maybe looks like a 1 ohm resistor (draws 12 amps at 12 volts).   It turns out that the most resistor placed in series with that 1 ohm resistor, which would draw the most power, is, another 1 ohm resistor.  As Iceng has already shown, the amount of power dissipated by that 1 ohm resistance would be 36 W.

That is indeed a beefy variable resistor.  I think when they get to that power rating, they start calling them "rheostats".  A variable resistor able to dissipate that much power will probably look something like this:

Jayefuu5 years ago
What's the 12VDC device?
tylervitale (author)  Jayefuu4 years ago
It's a massive peltier module.
What kind of control are you hoping to gain?

If your looking to control the voltage you'll need more then a variable resistor.
tylervitale (author)  mpilchfamily4 years ago
No, I don't care about the voltage. As long as the supplied wattage goes down, I'm fine.
iceng5 years ago
Most circuit resistor wattage can be determined by the size ( volume ).

Bigger resistors have the wattage screened on surface near the
ohmic value.

12 VDC at 12 Amps controlled by a a resistor.  Since you don't specify,
I select the resistor has to be able to vary the voltage to your unknown
load to 6 VDC, further this load is resistive and will only draw 6 Amps
at the lower voltage.

This variable Resistor = 6 V / 6 A = 1 ohm maximum.
Resistorpower = V × I = 6 × 6 = 36 Watt