From: https://www.howtogeek.com/109380/how-to-use-windows-8s-storage-spaces-to-mirror-combine-drives/

From: https://www.windowscentral.com/how-use-storage-spaces-windows-10

If you’ll be using mirroring or parity to protect against data loss, we recommend 
choosing ReFS for its file integrity protection features.

From: https://serverfault.com/questions/770472/mixing-disks-of-different-sizes-in-a-storage-spaces-pool

Also: https://social.technet.microsoft.com/wiki/contents/articles/11382.storage-spaces-frequently-asked-questions-faq.aspx#What_are_columns_and_how_does_Storage_Spaces_decide_how_many_to_use

Also:

These are the document you are looking for:What are columns and how does Storage Spaces decide how many to use? and Storage Spaces - Designing for Performance and Storage Spaces: Understanding Storage Pool Expansion

I will start by quoting two things from the articles provided:

"Note Storage Spaces in Windows Server 2012 R2 and earlier by default uses the largest column count possible given the number of disks you have and the resiliency type you select." a
"To extend [a storage pool], it would need the appropriate number of columns available to accommodate the layout of the disk." b
In other words: If you add physical disks to a storage pool, Windows keeps the amount of columns in said pool. Each row of disks you add must provide one disk per column.

From: https://blogs.technet.microsoft.com/askpfeplat/2013/09/24/storage-spaces-understanding-storage-pool-expansion/

Get-VirtualDisk can tell you many things about a virtual disk you’ve created within a storage pool.  
Given no parameters it will display the virtual disks it finds with some basic status information.  
However, you don’t get the full picture.  If you pipe the returned objects to Format-List, you’ll 
get much more detail…perhaps more than you want.

From: https://www.petri.com/what-is-microsoft-storage-spaces-direct

Also: https://docs.microsoft.com/en-us/windows-server/storage/storage-spaces/storage-spaces-direct-overview

S2D is a cluster that’s made of up servers with direct-attached storage (DAS). The supported disks include SAS HDDs and SSDs of the past, but S2D also supports:

    SATA disks: HHDs and SSDs can be very affordable
    NVMe: Non-Volatile Memory Express disks PCIe connected SSDs that offer huge amounts of IOPS, where one model I checked out offers up to 122,000 IOPS.