Article use

You are correct. No article is needed.

Blue Jay You are correct. No article is needed.

Thanks a lot for your help. But for some reason it also sounds possible with the article that was why I was confused!

Would the same apply if I used "nice" instead of "great" in that sentence?

You don't need an article with nice either.

Blue JayYou don't need an article with nice either.

Got it. Is the reason because we are implying that the individual's shape is nice/great, and not the fact that his shape is of of many other nice shapes?

In great shape is a standard phrase meaning in good condition, fit and healthy.
In a great shape suggests they are inside a wonderful triangle or octagon.

Sometimes people make a joke about this.

A, complaining: I'm so out of shape.
B: Not true! Round is a shape!

(In other words, they are a bit fat, so they are "round.")

If you are referring to someone's physical fitness, you'd say "good shape, great shape, out of shape - but not really "nice shape."

Blue JayIn a great shape suggests they are inside a wonderful triangle or octagon.

I can see that!

BarbaraPAIf you are referring to someone's physical fitness, you'd say "good shape, great shape, out of shape - but not really "nice shape."

Thanks for the clarification. I just used “nice” to see if I use other words would the same apply. “Nice” does not sound good here. Another good word would be “amazing shape”, do you agree?

Yes, I think you have the idea. Changing the word order doesn't change the fact that great shape (in this sense of the phrase) doesn't take an article.

Blue JayYes, I think you have the idea. Changing the word order doesn't change the fact that great shape (in this sense of the phrase) doesn't take an article.

I am glad I understand now. I can’t thank you enough for your help teacher. Thank you!