i squeezed, and i mean squeezed, an 18.5" imprint on our diamondback s one time. it wasn't easy either. i typically just tell people that we have a 16.5" limit. we have 21x23's and the frame wasn't the issue, just the stroke length of the press.
problem i was really running into was the design was a lot of halftone and the flood bar wouldn't reach the very top of the design. it would stop on the actual stencil and it would produce a line going across the design. had to raise the flood bar up and load the screen with ink so that there was some stuck on the squeegee. ended up working, but i wouldn't like to do it for every job.