Adding extra Disks to RAID 5

[will2rob]

I have a TS-853 Pro with 4 x 6TB WD red Drives set up in RAID 5. I am running out of space so I was planning to 2 x extra 6 TB drives in the spare slots and expand the RAID. However, I see that 8TB WD Red drives are now available. If I use those instead of 6TB drives, , will the RAID take advantage of the increased disk size and add 16TB into the RAID array or only 12 TB (as all the other disks are 6TB) Or should I set the new disks up in their own storage pool? The NAS is primarily used as a media server, so I just want to have the maximum space available for movies and TV shows in a single volume.

I can't afford 4 x 8 TB disks in one hit, so the plan is to buy 2 larger capacity drives rather than 4 x 6TB drives and buy another 2x8 TB drives later if the NAS will support 4 x 6Tb drives and 4 x 8 TB Drives in one RAID 5 array.

[Don]

Until all are replaced it will only use 6tb out of the 8.

[will2rob]

What about if I populate the 4 spare bays with 8 TB disks as a separate volume and RAID array?. e.g Volume consists of 4 x 6TB Disks, and Volume 2 consists of 4 x 8 TB disks, each volume set up as a separate RAID 5?. Will that give me more space than 8 x 6TB Disks in one volume/RAID?

[MrVideo]

Do the math:
.9x(3x6)+.9x(3x8) = 16.2 + 21.6 = 37.8
.9x(6x6) = 32.4