Something is big if it is many times larger than something else. The ship in the picture is much bigger than the little boat at the front of the picture. The opposite of big is small.
If something is much, much bigger than it should be, it is huge. The picture is of one huge pumpkin among a lot of normal-sized pumpkins.
Something is long if it goes for a considerable way in one direction. Something goes for a long time if it happens for an extended period. The lizard in the picture has a very long tail. The opposite of long is short.