In this narrow-field mode of MUSE, the CCD detector can resolve 0.025 arcseconds per pixel (arcsecond is a weird unit for angles used in astronomy). At the current distance to Neptune (according to wolframalpha: about 30 au = 4.5 bn km), this corresponds to about 500 km/px. Due to observing conditions, I think the real resolution was more like 0.07...0.08 arcseconds, so maybe it was 1000 to 2000 km/px.

I'm not sure if the focal length plays any role here. The resolution is usually limited by the telescope size (true for all telescopes, scales with 1/diameter) and atmospheric conditions (only relevant for ground based ones). At the distance of the moon (300,000 km), the physical resolution is 36 m/px and for the ISS (400 km) it is 5 cm/px.

If you want to play around with it, here's the formula: length_still_resolved = angular_resolution * distance

The angular resolution is 1.2 * 10^-7 (= 0.025 arcseconds converted to radian), distance and length_still_resolved have the same units.