A panel has to be sized to handle the load that will be imposed upon it. I do not understand what you mean by "high phase." Can you clarify the question?
If you've already got your panel schedule made up, and you're able to calculate the highest phase demand load, that would be a good method. If the A-B phase seems like it's higher than B-C or C-A, then that would be the one to use to size the panel.