This was big for bankers, up until the year 2008 when the global financial crisis hit.
What is the meaning of big in this sentence?
I'd say that this usage of big could be considered short for a big deal:
This was a big deal for bankers, up until the year 2008 when the global financial crisis hit.
TFD defines a big deal as:
Slang(n.) Something of great importance or consequence
In this sentence, big means important. Here is a definition: important, because of being powerful, or having a lot of influence or a serious effect.
Here are some other examples:
He had a big decision to make.
There's a big difference between starting up a business and just talking about it.
The big story in the news this week is the mayor's resignation.