What was the largest telescope ever fitted with an eyepiece

Realize that a large telescope would have a large prime-focus image, that would in turn necessitate a very extraordinary eyepiece to fully utilize.

This said, if you look at this image from the 200 inch Palomar telescope (apparently published in Life magazine):

You see an astronomer inserting a plate in the camera at the 200 inch prime focus. Close to his right thumb you see a vertical tube. This is an eyepiece that is used to examine a star at the edge of the prime focus image outside of the plate dimensions. The astronomer will watch this star through the eyepiece and manually adjust the tracking of the telescope to keep the star stationary. For hours. In the dark. With no source of heat. And no plumbing.

So we have an eyepiece being used in parallel with a photographic plate. It may be a stretch, but...