This reminds me of the visit I recently made to Berlin, for the European Planetary Science congress. On the last day there was a visit to the longest refractor in the world, the Archenhold Telescope, which has a focal length of 21m and aperture 68cm (f/31). The longest eyepiece they have is 100mm, giving x210. This is the power they nearly always use. The telescope is in regular use for public observing, particularly by school groups. The eyepiece is the green bit in the photo.