I think I have a similar choice from the above post; 2TB should be enough; it would take 3 to 5 years.
The reason why I chose 2TB is because if you chose 4TB, it would take 8 to 10+ years, and the problem is sometimes without maintaining this SSD, the lifespan might be shorter. That is why having 2TB is enough; at least you can expand again after 3 to 5 years once it's nearly full with brand new units rather than using 4TB that you don't know how long will serve you.
On the one hand, the only reason to buy an HDD with more storage capacity than is currently considered necessary is if you want to save money in the future. What I mean is that, due to all the hype surrounding AI, new HDDs with 4 TB or more of storage could be significantly more expensive in five years than they are now.
On the other hand, if you buy an HDD with excess capacity now, there’s no guarantee it will still be in working condition five years from now. For example, I have an old 20-year-old HDD that’s still in good condition, and I continue to use it as an additional HDD for backup storage; at the same time, I once had a 2 TB Seagate HDD whose reliability turned out to be not so great. That HDD lasted only 6 years, and then its magnetic read head failed due to a broken RCBO. At the time of the RCBO incident, both HDDs were powered on, but in the end, the older HDD survived the incident, while the newer Seagate HDD didn't. My point is that, in this regard, there’s no way to predict which hard drive will be more reliable in the long run and which won’t.