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Any size. I've got a 2A variac, and I've used it with 4 and 5A loads. A variac is a variable auto-transformer Transformers main enemy is heat. You can run 1.5-3X rated continuous load on a transformer until it gets too hot to hold your hand on for 5 seconds (turn it off at that point), and no harm will come to the transformer or load. 3X load may reach overheat in 5 minutes, 1.5X load may give over 5 hours.
P=IV P=Power(in W), I=Current(in A) V=Voltage You can use rated wattage and line voltage to calculate current.
Some don't like this, but some dimmer switches can be used as a variac in a pinch.
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