I was looking at this Nano ID page where they generate a 21 characters long ID and it would take 149 billions years to find a collision, so that seems secure enough.
However I found that most APIs use much longer tokens for authentication. For example, Dropbox uses a 64 characters long token (made of complete alphabet and numbers) and OneDrive uses a 1180 characters long token (also made of complete alphabet and numbers).
So I'm wondering why do they use such long tokens? And related questions, what's an ideal size for a token made of random letters and numbers?