the amount of space used will depend on how busy the VMs are during the operation, however, i have found whilst setting up the VCB modules with Networker in particular that you need to have a staging area on the proxy host = or > the largest VMDK you want to backup. in essence if you ahve a 500gb VMDK with 100GB of data in it then you need a "target" for staging the snapshot of 500gb or more. note though that this is the "VMDK" size and not the LUN that contains them.
so if you have a 1TB LUN on your SAN and you run 100gb VMDKs and you set the backup parrellism to 4 then you will require ~ 400Gb for your staging area.