not sure what stackexchange site is the best for this question.

I have a scanned jpg file with below properties and size 8.5MB

    pixel dimension: 2468 × 3484 pixels
    print size: 208.96 × 294.98 millimeters
    resolution: 300 × 300 ppi

I need to save the file as png (to have the pic transparent) while the size cannot be bigger than 4MB. The most important is that the size of picture must remain the same. I mean that the object size in the picture must stay the same.

Could anybody tell me what is used to define the size of the objects in the picture?