Ideally, I c/would be comfy with 500GiB, but--since it doesn't exist--I have only one choice (2TiB).
Since my Photos/iMessage databases are shared amongst the comps/phone, that's 310GiB right off the top.
Documents consume 50GiB; Backups 0,5GiB; Mail 0,2GiB
That leaves me with 1,65TiB of 'space', the entirety of which is currently unutilised.
OTOH, if I kept my Photos database to a minimum (e.g., copy-off-locally anything older than the past week), I could probably drop to the 200GiB plan: sadly, the USD7/mo is not that critical, and I let my laziness persist
I had/ve plans to upload some of the multi-GiB of data on-hand to maintain my strategy of multiple-multiple backups, but the time to measure the space between the tyre-and-the-road has been lacking for god-knows-how-long.
Local/Cloud storage is cheap, and there's only so many copies a person can mentor in a Day, so There It Is <smile>
Regards, splifingate