Sorry, I think you missed the point of the question. He's not asking about how to make a conventional print using an enlarger.
He's asking about how to make a large format negative from a small format negative, so that he can contact print the large format negative onto something too slow for an enlarger, like Azo or platinum or cyanotype, etc. Lots of people are doing this these days by scanning the negative and creating a new negative digitally using a film recorder or an inkjet printer and transparency film that is normally used for overhead projectors. He wants to know how to do it the traditional way in the darkroom, without using digital techniques.