How should an app clarify that you can choose the outcome size only during the first 2 of the 4-step process?

I am working on an app that synthesizes long texts, articles, essays. You paste in the text and the system starts synthesizing.

You can choose a size or length for the final format, say Short, Medium, Elaborate only during the first two steps of the four-step process. How do I make it clear that the user can only do that during the first two steps?

Yes, we could ask the question before the user clicks "Start" but we wanted to make it as quick as possible: upload and hit "Start"; then you can choose.