[Shiraz (Ray)]

the optical resolution of a scope in the lab is determined by the aperture and optic quality (eg the Rayleigh limit). For small scopes, the scope size and quality generally also determines the angular resolution that you get in the field (the ability to resolve small detail).

However, for a scope above about 6 inches, the optical resolution will be around an arcsecond or less, but the atmosphere will blur the scene by much more than that (eg 2 arcseconds or more), so, for any reasonably good optics, the resolution in the field will be determined almost entirely by the atmosphere. The seeing theory even has a term Fried parameter r0, which basically tells you the telescope size where the transition from scope-limited to seeing-limited performance occurs. For Australian conditions, this is about 80mm or less - provided you have appropriate sampling, your MN should have almost as good a resolution as the 3.9m AAT most of the time and a 10inch scope will not be any better.

from Wiki: For telescopes with diameters smaller than r0, the resolution of long-exposure images is determined primarily by diffraction and the size of the Airy pattern and thus is inversely proportional to the telescope diameter. For telescopes with diameters larger than r0, the image resolution is determined primarily by the atmosphere and is independent of telescope diameter, remaining constant at the value given by a telescope of diameter equal to r0. r0 also corresponds to the length-scale over which the turbulence becomes significant (10–20 cm at visible wavelengths at good observatories)

The above applies to long period imaging (DSOs). However, where the imaging is fast enough to take advantage of short bursts of good seeing (visual or high speed planetary imaging, or adaptive optics), bigger apertures still have an advantage.

also note that seeing measures are in angular units, so focal length does not come into the equation.