OK, I'll take a stab at this--
Input: 75,000VA / 1.73 / 480V = 90A. Call it #3 or #4, depending on whether you play by the "next larger standard breaker size" rule.
Output: 75,000VA / 3 / 120V = 208A. If you call this 200A, it will take 3/0 cable. I don't know if the rules would allow fusing this at 225A; if so, you'd want to use 4/0.
The above assuming 75 degree wiring.
I would imagine that either the input or the output fusing is supposed to limit the TX to 75 KVA; I don't know what the rule is for this. In a vacuum, I'd say a 90A breaker on primary feeder would do the job.
