I am looking to get 1phase 240/120 temporary power from a 3phase 480V power source for construction purposes. My electrical contractor suggested a 5kVa transformer but didn't give details. Question is: 5kVa transformer would technically be good for around 20 amps at 240Volts right?

rogadoga,
Yes, 5000 VA / 240 V = 20.83 Amps at 240 volts.
If you could describe your situation a bit more I might have other
recommendations...
What do you need the 120V or 240V for?
Can you describe the general size of this project?
The number of people working on site each day?
When could you install a more permanent electric service?