The print size is determined by the focal length of the lens and the height (distance between negative and easel). The shorter the focal length, the larger the magnification. The larger the height, the larger the magnification.

The focal length of the lens is a guide to what size negative can be used with it. Many 135mm lenses "cover" a 4 x 5 negative out to the corners, while a few only "cover" a smaller negative. By "cover" I mean that when you look at the print, the corners of the image aren't darkened, and the details of the image at the corners are sharply resolved.