voljega I think you misunderstood the problem. He has already bought a new 64 GB card.
SD-cards, even if they are sold with the same capacity, can differ in the usable space. So this, in my opinion, is the reason for this post. But you are right bying a larger sd-card for batocera is always a good idea.
So if he has a 56,8 GB image means that he is using an image that was not created or distributed by batocera team at all. These images usually contain copyprotected software. There will be no support by the batocera team (which I totally agree to).
Als these images are “packed” with games I think shrinking will not work.