If you talk about the magnitude of something, you are talking about its great size, scale, or importance.

Magnitude is used in stating the size or extent of something such as a star, earthquake, or explosion.

You can use order of magnitude when you are giving an approximate idea of the amount or importance of something.

In mathematics, if one amount is an order of magnitude larger than another, it is ten times larger than the other. If it is two orders of magnitude larger, it is a hundred times larger.