What is the current long-term best hard disk size?

Hi,

With "best" I mean:

1. this size will be available by all providers (=competition) and has the chance of
2. being the most cost-effective in cents/GB for the next couple of years and
3. has a technology that is reliable and proven.

When I started with unRAID back in 2011 2TB drives were the only choice. Relatively soon, 4TB drives became the standard and were the ones with the lowest cost per GB until now. I mostly use WD reds now after HGST was bought by them with some bad experiences with Seagate and Toshiba in-between.

At the moment it seems that higher capacities start to challenge the 4TB prices. WD currently has an attractive Black Friday deal for their 18TB Pros. I am wondering if it is the time to swap out the drives starting with my two parity drives. But is 18TB the "right" size to adopt as the new standard?

I know that unRAID does not care about drive size as long as the parities are the biggest ones. But with so many new large capacity drives that add yet another TB or two on top I don't want to constantly swap out the parity drives.

So I wonder what the your opinion is about:

1. What hard disk sizes are, from a technology perspective, the most reliable ones? I always thought that doubles 2-4-8-16 would be a a logic. 

2. Which disk size has the potential to become the new "lowest cost per TB" leader for the next years?

I researched this but could not find any clear answers. BackBlaze thinks that the cost will come down to 1ct/GB in 2025: https://www.tomshardware.com/news/backblaze-expects-one-cent-per-gb-hdds-by-2025. They seem to use 16TB drives now but the reports says that the 1ct will be achieved by the 22/24TB drives. 24 would be doubles again. Interim sizes like 16, 18 or 20 would be shorter-lived in-betweens!?

So wait for the 24TB? The prospect of parity checks/rebuilds taking 3.5 days with 24TB (my 4TB take 14 hours) scares me a bit.

Overall, the WD Reds Plus 12TB seems to be the current non-deal sweet spot and every provider has 12TB drives. So is 12TB a good size to go for now?

Appreciate you insights and wisdom. 

Thanks!

Tazman