Not sure what you mean about not knowing the capacity, never seen a mini sd card that did not have the size printed on the card itself.

If the card should have room for pictures, but the camera says it does not, then its possible the card has been corrupted somehow.

After copying the pictures over to the computer I would look through the cameras menu and see if there is an option to format the card, that will reinitialise the card, wipe it and set up the folders needed by the camera.

Then try again and see if the camera responds correctly.