Well when applied to visual telescopes, the practical "magnification" limit, which is what you asked for, would be that angular magnification, which magnifies the diffraction limited image of the telescope up to the resolution limit of the eye.
Now a telescope of about 4.5 inches aperture resolves about one second of arc, so a one inch telescope would resolve about 4.5 seconds of arc.
The human eye in a relaxed state can resolve about one minute of arc, so it would take a magnification of about 33.3 to bring the star image up to one minute of arc. But that is an extreme resolution for the eye and is strainful, so about 1.5 minutes of arc is a more robust viewing condition, which means a magnification of about 50 (angular mag) for a one inch telescope.
Lots of telescope making books, have suggested 50 per inch of aperture as the maximum useful visuaual magnification. For non visual sensors, you could go above that, as for example with your mega-telescopes.