What is the wire size in this service. And what size triplex does it use

Don, this is an unusual service size...
Can you tell me the voltage? Single phase or three phase?
Is this the rating of the meter socket by any chance?
Is this commercial or residential?
What do you intend to do with the triplex?

I apologize but it's sometimes inappropriate to give simple
Answers based on assumptions... Please tell us more about your
Situation.

A 320/400A service is very typical in residential settings.

Until you can provide more detail there is not much we can accurately tell you.

Why are you asking?
And why ask about triplex? This is typically utility territory.