MRO’s HiRise camera has a 0.5m primary mirror and a per-pixel resolution of ~30cm from an altitude of 200km. For an identical orbit, and ignoring any atmospheric effects, what mirror size would be needed for a resolution of 0.25 m/px?
The question does not make optical sense (as currently asked) and does not have a numerical answer.
Aperture size affects the diffraction limit, but pixel size is an independent choice, and you need the ratio of the distance to target and telescope focal length, not the aperture size, to get resolution in meters/pixel.
One might assume that the designers set the pixel size to roughly something like one half the diffraction limit, but that's wavelength dependent which is a big factor for some systems that operate over a very wide range of wavelengths.
It also depends on the shape of the aperture, and type & size of obstructions
If you'd like to update the question I'm happy to update this answer.