What size of step down transformer I should use if the existing 480volts from the Main Panel is 70amps? I can use 70amps as my step down size transformer?

What does your electrician suggest?

You could install any transformer 45 kVA or larger...
But "don't" do that before you have some idea of the load or equipment to be connected to it.
If this is for future use and the future load is unknown it would be more appropriate to size the overcurrent device to the transformer, not the other way around.