You may need to power down the VM to adjust it, but while in the vsphere management client, where you see all of your VM's, right-click on the one you wish to adjust, select "edit settings" -> go to the hardware tab and you'll see a variable for "provisioned size". You can adjust it up as long as the hardware in the host has sufficient capacity. There should be a "maximum size" noted on this screen as well that is determined by the hardware and other vm's. Best practice in my experience is to leave 12-15%+ available...