Best way to estimate the size of a parking space (Python, OpenCV)

Question

I am trying to determine the size of a parking space using a single camera.

For example, a parking space is on one side of the road between two vehicles and the camera is now looking from the side.

What is the best way to solve this? I have read that there is the Circle based Marker option where you detect a known length and use that to get the distortion out of the camera, then use the pixels and the deviation to determine the actual length.

What is the best way to solve this with Python or is there an alternative that is better?

is there any Github repo, for a guide?

Edit: i don't have an actual image but i did a sketch for example. The Camera Position could be variable, so it's not a fixed position