I used to believe that Allah gave some people the power to prophesy so that they could prophesy. They could not have prophesied without Allah's help. Is believing this minor shirk or major shirk?

If you have this belief about the prophets, then it is not Shirk, rather it is a part of our faith because the prophets know only as much about the unseen as is acceptable to Allah. 

And if you have this belief about someone or a common man that Allah gives him the knowledge of the unseen, then this is a wrong thing from which you should repent because the knowledge of the unseen has not been given to anyone except the prophets and they too have only as much as is acceptable to Allah, but still this thing is also not Shirk.

Shirk means associating someone else with the essence or attributes of Allah, such as acknowledging or believing that someone other than Allah is worthy of worship. It means that a person denies the oneness (aloneness) of Allah and shares it with others. For example, saying "So-and-so is also the creator" or attributing to someone else an attribute of Allah, such as "helper," is shirk.

And there's no such thing in your question.

Jazakallah khair