Merlin,
for what you're doing, the calculation is simple. You can do it yourself.
Voltage x amperage = Watts. transformers step voltage up or down but watts in always equals watts out. Watts are also known as VA (or volt amps).
(somebody's going to correct me and say that VA is different than watts and they're right but for something this simple, you don't need to worry about that)
So you want 120 volts and 10 amps. 120*10 =1200 Watts or 1.2 KVA.
Just for fun, if we need to know the input amperage at 480V we still use V x A = W. or 480 x ? = 1200. We get 2.5A.
So 2.5A at 480 volts input give us 10A at 120V output.
