"Magnitude" can suggest something large, or "strength".
The magnitude of this scandal is mind-blowing.
The magnitude of last night's earthquake in Sheffield is not yet known.
"Amount" is a term that can be used with any size-specific term.
He has a very small amount of money.
The Prime Minister has a huge amount of money.
You only need to add a tiny amount of salt to this dish.
He put a reasonable amount of chilli in the pot.