Performance Limits for Telescopes
There are several factors that influence the performance limit, as measured by maximum visual magnitude and resolution, primary among these is the aperture of the telescope:
Aperture (inches) Maximum Magnitude Theoretical Resolution  
3.1" (80mm) 12.2 1.50 arc-seconds
4" (100mm) 12.7 1.20 arc-seconds
5" (125mm) 13.2 0.95 arc-seconds
6" (150mm) 13.6 0.80 arc-seconds
8" (200mm) 14.2 0.65 arc-seconds
10" (250mm) 14.7 0.50 arc-seconds
12.5" (320mm) 15.2 0.40 arc-seconds
14" (355mm) 15.4 0.34 arc-seconds
16" (400mm) 15.7 0.30 arc-seconds
20" (500mm) 16.2 0.24 arc-seconds