Sure you can, to the extent that the size of a dataset is defined at all--quotas and reservations can be changed at will.
@pralon, in addition to describing your hardware and FreeNAS version, what's the output of 'zfs list'?
here is the output:
[root@freenas ~]# zfs list
NAME USED AVAIL REFER MOUNTPOINT
storage 737G 1.05T 200K /mnt/storage <-- a lot of free space, but couldnt create new dataset
storage/.system 4.93M 1.05T 168K /mnt/storage/.system
storage/.system/cores 412K 1.05T 412K /mnt/storage/.system/cores
storage/.system/samba4 3.55M 1.05T 3.55M /mnt/storage/.system/samba4
storage/.system/syslog 840K 1.05T 840K /mnt/storage/.system/syslog
storage/Accounting 10G 99.0G 992M /mnt/storage/Accounting
storage/Collector 1.00G 0 1.00G /mnt/storage/Collector
storage/ERPBackUp 287G 12.9G 287G /mnt/storage/ERPBackUp
storage/ERPPatch 200G 144G 55.9G /mnt/storage/ERPPatch
storage/ERPTest 200G 49.3G 151G /mnt/storage/ERPTest
storage/ICT 39.2G 70.8G 39.2G /mnt/storage/ICT
and system info:
System Information
Hostnamefreenas.PRALON Edit
BuildFreeNAS-9.2.1.5-RELEASE-x64 (80c1d35)
PlatformIntel(R) Xeon(R) CPU L5520 @ 2.27GHz
Memory4075MB
System TimeSun Mar 29 19:13:35 WIT 2015
Uptime7:13PM up 272 days, 20:33, 0 users
Load Average0.00, 0.02, 0.02