Well you might be hitting the limits “by design” of the default VMware configuration.
You need to check what is your store block size:
Configuration -> Storage -> Select the one you’re talking -> Look at “Details” the below pane “Block Size”.
By default, when data stores are created, their block size is 1 MB. This will only give you a virtual hard drive with a maximum of 256 GB. In order to get larger VHD’s, you need to delete data store(s) and recreate it/them keeping the following assumptions on how block size implicate maximum data store size:
Block Size Max. DS Size
Of course this assumes that by <b>deleting a data store you will loose everything you have in it.</b>. So, if you already have virtual machines running there you need to back them up first.
For this task a second ESX (i) would be useful to minimize downtime, or if you have a cluster with vMotion the task will be even easier with no downtime…