Use the command vmkfstools -X 18000M SANx:disk_d.vmdk
where 18000M is the size of the new disk in Mb (not the size you are increasing by but the end size), SANx is the SAN label and disk_d.vmdk is the name of the file. Make sure you use uppercase X as well.
Once extended though you will need to extend this inside of windows. If it is a dynamic disk this is easy or you may have to use the diskpart utility if W2K3.