calibration gives you two things: an estimate of the lens distortion, and an estimate of the focal length, which combines the field of view and the pixel size of the sensor in a single value (no relation to “focus”).
you could assume the lens distortion to be zero. you could also just calculate the focal length if you have both the size and distance of a known object (yard stick or whatever).
with a single camera (and the focal length), if you want to measure the size or distance of an object, you’ll still need to know either the distance or the size of the object, respectively. “similar triangles”, the math is called.