Subjective/YMMV just as the decision on big versus small knots. I have a 28mm knot and 57mm loft M&F that I love. Other big brush users consider 57mm to be too much. You need to decide based on your preferences rather than polling for random opinions. If you don't know what your preference is then you'll have to discover it via first hand experience.To those who love a big brush. what loft to use with a 30mm knot?
Still subjective/YMMV. I face lather and, as I said, the loft on my brush is 57mm. Many face latherers seem to prefer shorter lofts. However, that doesn't mean that all (or the OP) do.i don't use large brushes, but what are you using it for: face or bowl lathering? This would determine the length of loft you would want to use.