Online capacity expansion can be done on live virtual disk. It is not recommended to do it during working hours as there will be performance impact during the process of online capacity expansion.  You can use 600GB drives instead of 300GB. Please note you can only consume 300GB space of those 600GB drives in the virtual disk as other drives on the RAID 5 array is 300 GB.  You will get additional space of 600GB when you expand the current RAID 5 with these 2 new drives. You can add both disks to the existing RAID 5 at same time.  
Thanks for your input everyone. I have a PE T320 with a PERC H710 RAID controller running Server 2012r2.  Currently configured as RAID1/5 I want to expand the current RAID 5 array by adding an additional 2 disks and have some questions. (online capacity expansion), i've read  'how to' posts on the process and it looks relatively straight forward and i understand that this can be performed on an existing and live RAID5 array, but is suggested to schedule out of working hours? i do have a couple of other specific questions which i cannot find answers to, an my account manager has yet answered.  Many thanks for your help Jon  
Poweredge T320 /PERC h710 Online Expansion of RAID 5 array