From their Wikipedia page:

"Before the 2008 collapse and the general financial crisis, RBS Group was very briefly the largest bank in the world and for some time was the second largest bank in the UK and Europe"

http://en.wikipedia.org/wiki/The_Royal_Bank_of_Scotland_Grou...

Of course, it depends how you decide on the size of a bank... Also they had been on an insane binge of acquisitions, including the ABN Amro deal which was possibly the worst acquisition in history (paying tens of billions with one ring folder of information for due diligence - what could possibly go wrong!).

Interesting. They didn't even have a proper investment banking arm, just primarily fixed income, and not much of a bank network outside of the UK.

Here is a good documentary about the fall of RBS and ex-Sir Fred "The Shred" Goodwin:

http://www.youtube.com/watch?v=fg3K-S5GqBo