Oversize the transformer by 25% from the demand. It won't hurt to go bigger, but it's not necessary.

THEN, oversize the circuit breaker 20% from the transformer full-load amperage. Fun fact: most circuit breakers are only rated to guaranteed not tripping at 80% of their rated load. The magnetizing current is actually way higher than the full load rated current, but decays before the circuit breaker will trip (slow).

What voltages are we talking about?