Set a quota of 1 Gigabyte for the user 4000187 (the values are in kilobytes, so 1048576 kilobyte are 1024 megabytes which corresponds to 1 gigabyte):
xfs_quota -x -c 'limit bhard=1048576k 4000187' /var/backup
Or in bytes:
xfs_quota -x -c 'limit bhard=1073741824 4000187' /var/backup
Read the quota information for the user 4000187: