The size of the card depends on the length of the shot. 1080P video is about 40M/ sec. So:

1: The speed of the card must be 40M per second to shoot 1080P video normally.

2:720P video is about 15M per second. You can shoot ordinary cards.

The speed of the card is expressed by class. Generally, the cards used for video are above CLASS 10.

If you use a CLASS4 card, you can only take photos, and continuous shooting is not enough.

On the front of the card, the card speed will be indicated, and the Arabic numerals in a circle will be indicated. Those without card speed are basically non-genuine, which is the so-called "fake" card. This kind of card is generally not used in cameras.

If you find that your card is fake, or it is a class 4 or 6 card, then change it.