How to find the actual size of an existing object from the pixels of an image?
I really am no expert in cameras (nor in OpenCV in which I'm pretty sure everything can be handled automatically et efficiently without doing calculations yourself). But without knowing the distance between the lens and your object, that will be difficult.

Actually, knowing the focal length of your camera and the distance from the nodal point to the image plane, you can deduce the distance from the nodal point to the object. Then just use Thales' theorem and you can do exactly what you want.

The problem of this method is that it relies on how well the focus is done on the camera. And because of the varying depth of field of your camera, the magnitude of the error will vary too. AFAIK, this method should work pretty well for short distances. For longer ones, that can be very difficult as the focal point is near infinity and the depth of field is really large.

As an alternative, you can put an object of known size next to the object which size you are measuring. Thales' theorem again and you get your data.