I power it with a Lippo battery 7.4 votl 2400 mA. I use two individual motor drivers L298N, one for each stepper motor, 1 arduino uno and 2 stepper motors from dvd drivers.
My problem is that the motors heat a lot. I guess I should use a resistor to cool them down but I don't know how to calculate the value (if that is the solution)
You would normally limit the motor current by controlling the time for which each motor coil is energised. Current takes time to build up, due to the coil's inductance.
Too hot for touch for a human is around 50C-60C. Most permanent magnet motors can run at least up to 70C safely, but don't go farther if you're not sure since that's when certain types of magnets begin to demagnetize.
If you are able to hold your finger on the motor 2-5 seconds after it has shutdown, you are probably fine.