It used to be that the biggest free account you could have was 10GB (8GB max of referrals added to the 2GB free account). But as of yesterday Dropbox has doubled the referral limit, and you can now get up to 16GB free storage added to your 2GB plan. If you’re a paying user, the referral limit is double-doubled to 32GB (!).
On top of all that, the referral bonuses are retroactive. So check your storage because you may have some extra space that wasn’t there yesterday.