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 Business team. For Business teams, the size of the shared folder only counts once against the team's 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 or Professional.
Dropbox Business is 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 Business team 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.