What's the largest size drive I can use when running version 5.0.6? Not sure if it's worth to buy anything over 4TB right now since I can grab a couple for like $140 each.

Any size you want.  Any system that supports drives > 2TB will support any size that's available now or in the foreseeable future ... 4TB, 8TB, 12TB, 20TB, etc.

 

The largest drive currently available is the 8TB Seagate Archive unit.    This is a shingled technology drive; but it works fine in UnRAID.    I've posted a sticky that outlines the mitigations Seagate has made in their design that allow it to perform just fine, despite the shingled bands:  http://lime-technology.com/forum/index.php?topic=39526.0

... One other thought:  I presume you're aware that your parity drive must be as large as any other drive in the system => so another way to answer your question is "Nothing larger than your parity drive".    But clearly you could upgrade the parity drive to a larger drive if you want to 

This is why I may just stick with using 4TB drives. I have several 2 TB and 1 TB drives still to replace and my parity drive already 4TB. Once I switch to 4TB, I will probably have a couple years of space ready to go. I do have a lot of purging to do. But one or two drives are acting up and want to replace right away, I just bought 3 4TB HGST drives. 3 Year Warranty. Not as good as ny WD Black 5 year.

 

If simply upgrading your smaller drives to 4TB provides all the space you need for the foreseeable future, that's not a bad idea.

 

One thing to consider, however => upgrading your parity to an 8TB drive would, at the same time, provide you with a spare 4TB drive (the old parity drive) you could either add to the array (if you have space) or use to upgrade one of your older 1TB drives to 4TB.    ... so the actual cost of upgrading parity to 8TB isn't the cost of the 8TB drive -- it's the cost DIFFERENCE between that and a 4TB drive, which isn't much over $100  [The 8TB archive drive is ~ $260 if you watch for sales.]

 

If simply upgrading your smaller drives to 4TB provides all the space you need for the foreseeable future, that's not a bad idea.

 

One thing to consider, however => upgrading your parity to an 8TB drive would, at the same time, provide you with a spare 4TB drive (the old parity drive) you could either add to the array (if you have space) or use to upgrade one of your older 1TB drives to 4TB.    ... so the actual cost of upgrading parity to 8TB isn't the cost of the 8TB drive -- it's the cost DIFFERENCE between that and a 4TB drive, which isn't much over $100  [The 8TB archive drive is ~ $260 if you watch for sales.]

 

Thanks for the info. I know how quick prices can drop as soon as some competition comes around so I'm just going to stick with swapping about (a bad drive I just found) and another 1TB out. Once I get the bad 2TB drive replaced back I'll just take out another 1TB. So I'll have gained quite a bit. It is amazing how long I've had this system and how reliable it has been. If I were to get a Synology to handle the amount of tera bytes I have, I would have spent A LOT more money. A LOT more. And the hard drive cages I bought in 2011 are still working perfectly like day one. Very quality hardware.

 

Hey! That's my case! (except my handles are red)