A quick update, I uploaded a new Python off-nadir resolution calculator to my GitHub:
It only works for low-oblique images (theoretically it would also work for nadir images if you set the pitch of the camera to zero). It calculates the pixel resolutions for the near field (closest to the camera), mid field (at the principal point), and the far field (furthest for the camera). It will also tell you the area that each image covers.
If your interested in how it works head over to the GitHub repository.