I have successfully set the value of 'Disk space' and 'traffic' field to unlimited for domain project.domain.com, under Plesk >> statistics. Please refer the attached screenshot. 
In order to set it, I took below steps: 
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Before doing any changes I took backup of PSA database: 
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[root@server ~]# mysqldump -uadmin -p`cat /etc/psa/.psa.shad 
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1. Logged in to plesk database: 
[root@server ~]# plesk db 
2. Checked domains table in order to find that domains 'project.domain.com': 
mysql> select * from domains where name like '%project%'\G; 
*************************** 1. row *************************** 
id: 1 
cr_date: 2014-08-19 
name: server.domain.com 
displayName: server.domain.com 
dns_zone_id: 1 
status: 0 
htype: vrt_hst 
real_size: 541623 
cl_id: 1 
cert_rep_id: 0 
limits_id: NULL 
params_id: NULL 
guid: 6a1f3605-c874-4f54-a23c-84f31828b7a3 
external_id: 
adminDescription: 
resellerDescription: 
description: 
overuse: false 
gl_filter: on 
vendor_id: 1 
webspace_id: 0 
webspace_status: 0 
parentDomainId: 0 
permissions_id: 0 
*************************** 2. row *************************** 
id: 2 
cr_date: 2014-04-29 
name: project.domain.com 
displayName: project.domain.com 
dns_zone_id: 2 
status: 0 
htype: vrt_hst 
real_size: 5539550985 
cl_id: 1 
cert_rep_id: 0 
limits_id: NULL 
params_id: NULL 
guid: 02623dab-cb68-4059-bc31-b3da01d5410a 
external_id: 
adminDescription: 
resellerDescription: 
description: 
overuse: false 
gl_filter: on 
vendor_id: 1 
webspace_id: 0 
webspace_status: 0 
parentDomainId: 0 
permissions_id: 0 
2 rows in set (0.00 sec) 
ERROR: 
No query specified 
3. Checked subscription table: 
mysql> select * from Subscriptions; 
+----+-----------+-------------+--------+--------------+--------+--------------------------------------+-------------+ 
| id | object_id | object_type | locked | synchronized | custom | uuid | external_id | 
+----+-----------+-------------+--------+--------------+--------+--------------------------------------+-------------+ 
| 1 | 1 | domain | false | true | false | 14589f07-eb83-93f6-b4b8-3a532969f363 | | 
| 2 | 2 | domain | false | true | true | 9e94ba8c-c3b1-f8ca-f77c-aecbe4871720 | | 
+----+-----------+-------------+--------+--------------+--------+--------------------------------------+-------------+ 
2 rows in set (0.00 sec) 
4. Checked SubscriptionProperties table with id 2: 
mysql> select * from SubscriptionProperties where subscription_id=2; 
+-----------------+--------------------------------------+-----------+ 
| subscription_id | name | value | 
+-----------------+--------------------------------------+-----------+ 
| 2 | aps_bundle_filter_id | 1 | 
| 2 | default_server_mysql | 1 | 
| 2 | default_server_postgresql | none | 
| 2 | informer_files | false | 
| 2 | limitsId | 2 | 
| 2 | outgoing_messages_domain_limit | default | 
| 2 | outgoing_messages_enable_sendmail | default | 
| 2 | outgoing_messages_mbox_limit | default | 
| 2 | outgoing_messages_overlimit_percent | 0 | 
| 2 | outgoing_messages_subscription_limit | default | 
| 2 | OveruseBlock | true | 
| 2 | OveruseNotify | false | 
| 2 | OveruseSuspend | false | 
| 2 | permissionsId | 3 | 
| 2 | phpSettingsId | 1 | 
| 2 | unpaid_website_status | suspended | 
+-----------------+--------------------------------------+-----------+ 
16 rows in set (0.00 sec) 
5. Checked the current value for subscription in Limits table (limitId = 2): 
mysql> select * from Limits where id=2; 
+----+------------------------+-------------+ 
| id | limit_name | value | 
+----+------------------------+-------------+ 
| 2 | disk_space | 10737418240 | 
| 2 | disk_space_soft | 5368709120 | 
| 2 | expiration | -1 | 
| 2 | expiration_soft | NULL | 
| 2 | max_box | 1 | 
| 2 | max_db | 2 | 
| 2 | max_dom_aliases | -1 | 
| 2 | max_maillists | 0 | 
| 2 | max_mn | -1 | 
| 2 | max_site | 1 | 
| 2 | max_site_builder | 0 | 
| 2 | max_subdom | -1 | 
| 2 | max_subftp_users | -1 | 
| 2 | max_traffic | 32212254720 | 
| 2 | max_traffic_soft | 10737418240 | 
| 2 | max_unity_mobile_sites | 0 | 
| 2 | max_webapps | 0 | 
| 2 | max_wu | -1 | 
| 2 | mbox_quota | 1073741824 | 
+----+------------------------+-------------+ 
19 rows in set (0.00 sec) 
6. updated the value of the parameters 'max_traffic' and 'disk_space' to unlimited: 
mysql> update Limits set value='-1' where limit_name='disk_space' and id='2'; 
Query OK, 1 row affected (0.03 sec) 
Rows matched: 1 Changed: 1 Warnings: 0 
mysql> update Limits set value='-1' where limit_name='max_traffic' and id='2'; 
Query OK, 1 row affected (0.13 sec) 
Rows matched: 1 Changed: 1 Warnings: 0 
7. Checked Limits table value again : 
mysql> select * from Limits where id=2; 
+----+------------------------+-------------+ 
| id | limit_name | value | 
+----+------------------------+-------------+ 
| 2 | disk_space | -1 | 
| 2 | disk_space_soft | 5368709120 | 
| 2 | expiration | -1 | 
| 2 | expiration_soft | NULL | 
| 2 | max_box | 1 | 
| 2 | max_db | 2 | 
| 2 | max_dom_aliases | -1 | 
| 2 | max_maillists | 0 | 
| 2 | max_mn | -1 | 
| 2 | max_site | 1 | 
| 2 | max_site_builder | 0 | 
| 2 | max_subdom | -1 | 
| 2 | max_subftp_users | -1 | 
| 2 | max_traffic | -1 | 
| 2 | max_traffic_soft | 10737418240 | 
| 2 | max_unity_mobile_sites | 0 | 
| 2 | max_webapps | 0 | 
| 2 | max_wu | -1 | 
| 2 | mbox_quota | 1073741824 | 
+----+------------------------+-------------+ 
19 rows in set (0.00 sec) 
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