If you talk about the magnitude of something, you are talking about its great size, scale, or importance.
immensity , extent , enormity
smallness
An operation of this magnitude is going to be difficult.
一个这么大的手术会很难的。
These are issues of great magnitude.
这些是非常重大的问题。
No one seems to realise the magnitude of this problem.
看上去没有人意识到这个问题的严重性。
order of magnitude
You can use order of magnitude when you are giving an approximate idea of the amount or importance of something.
America and Russia do not face a problem of the same order of magnitude as Japan.
美俄两国所面临的问题并没有日本的那么严重。