about 6x9 from 35mm (old Tri_X) for me (and the grain is visible at that). Never shot any in MF (645) to know my limits there.
regardless, I just wish people would stop claiming 'grainless' Tri-X prints, especially in the magnifications quoted... They might be satisfactory, look great, etc, but they sure ain't grainless! I'm still waiting for someone to send me a 'grainless' Tri-X neg... I doubt it will ever turn up!!!