To be more accurate, eggs are single cells when they are just laid or unfertilized. Because they are considered single cells, they are the largest cells on earth! Of course, this definition isn't exactly cut and dry.
If the shell of the egg is considered the cell membrane and everything else inside is considered the cell, it can be said that it is a single cell. Then, the chicken egg deserves its reputation as the largest cell. If, however you consider the yolk and the albumen, which is the egg white, separate structures, then eggs are not a single cell.
Instead, they are made up of a few structures that are different cells, so that the chicken egg is not actually the largest single cell. In general, eggs are considered to be single cells by the scientific community. It's funny to imagine how cells, which are generally too small to be seen, could be so large that they fit in the palm of your hand.
Just think about how large an ostrich egg is! So what do you think, is the egg a single cell?