When you created your VMFS datastore, what size was your block? If you used the default 1 MB block size, the biggest disk you can create is 256 GB. With a 2 MB block size, you can create a 512 GB disk size, so you will need the 2 MB block size in your vmfs to create the size of disk you want on your vm. So, you can either re-format your vmfs, or add two 256 GB disks instead to your vm.
-KjB
vExpert/VCP/VCAP
vmwise.com / @vmwise
-KjB