Vmdk size matters only during the first backup as it requires full disk scan. All subsequent incremenrals grab only data that has changed since the last backup. For better space savings I'd recommend you to enable the BitLooker
Aside the fact that Windows software RAID might be a real P.I.T.A., software RAID inside guest OS won't do any good anyway. If you want to make sure that your vmdks won't get corrupted due to a hardware failure then simply setup hardware RAID on a hypervisor level.