The size of a shared folder will be counted against the quota of every member of that folder, unless each person is on the same Dropbox Standard, Advanced, or Enterprise team or Dropbox Family plan. For Standard, Advanced, and Enterprise teams and Family plans, the size of the shared folder only counts once against the shared quota.
For all other users, your Dropbox storage quota is calculated by adding up the total amount of data in your Dropbox folder, including all shared folders and files collected from file requests. As a result, the owner of a Dropbox Basic 18 GB (2 GB + bonus storage earned through referrals) account will not be able to collaborate on a shared folder larger than 18 GB. One reason this policy exists is to prevent people from giving themselves unlimited space by stacking Basic accounts with shared folders.
If you need more space, consider upgrading to Dropbox Plus, Family, or Professional.
Dropbox team accounts
Dropbox Standard, Advanced, and Enterprise are designed for projects, groups, and other organizations that want to share the same Dropbox storage quota with all of their members.
If you're on a Dropbox team account and you share a folder with someone else on your team, the shared folder uses only the team's aggregate storage quota. However, if you share a folder with someone not included in your team, the folder uses both your team's space and space on the account of the person you shared with.
Learn more about Dropbox Standard, Advanced, and Enterprise.