Great news.
The light bulb trick is for initial testing in case of short circuit.
Purpose is to limit the voltage and current through the device under test.
So for me I would size the light bulb to be 20% of the full load power of the device under test.
So, if for example the DUT is 20 watts, so the light bulb should be 100W [around to your discerning ability of shorts].
The sizing is not for the light bulb to lights up or maybe abit.
the light bulb must be incandescent light bulb.
you must wired in after the isolating transformer [after any device that consume power] and in series to the DUT.
The isolating transformer must have the power sufficient for the DUT X 2.
I have 1500VA isolating transformer to meet the occasion.