I need more than 3 GB per user.

If you are using the Free plan or a Trial plan, you may upgrade to a paid subscription. Each user you add to the subscription above your original 10 will increase your total YouTrack disk space by 3 GB.

If you are using a paid subscription, you may upgrade your subscription by adding new users, and each user in your subscription adds 3 GB to your total YouTrack Disk space available.