This tutorial explains how to change an email account's space limit in DirectAdmin, setting it to use the maximum available.
Access the Email Management area
1 - > In the DirectAdmin side menu, click Email Management, then
2 - > Select Email Accounts.
3 - > Locate the desired email account, click the three dots icon next to the account and select
4 - > Click on Change Limits.

3. Set the maximum size
1 - > Check the Maximum box in the Email size (MB) field and
2 - > click Save to apply the change.

Notes:
The maximum quota depends on the total space available in your hosting plan.
Setting it to Maximum allows the account to use all the free space available in the plan.