Default behavior of Storage Spaces Direct results in what capacity?

1) There are many ways to skin a cat (c) ... Depending on how you'll chop your disks and how you'll configure system (HDDs + SSDs for cache Vs. mirror accelerated parity etc) you'll get different usable capacity.

2) It's a bad idea to use S2D on only two nodes. It's fragile and every time you'll have one host down for whatever reason (maintenance, unplanned downtime etc) and you'll have just ONE disks failure in the alive one - your cluster will went South.

The considered setup will result in 6 TB of usable capacity and 1 TB of cache per node.

You can always check it with this calculator:

https://s2dcalc.blob.core.windows.net/www/index.html

I would also never trust 2 node s2d to keep mission-critical data. Unfortunately, such a setup is still from perfect and seems not thorougly tested. For the similar case, I tend to recommend a third-party Starwind free:

https://www.starwindsoftware.com/starwind-virtual-san

Hope to see s2d beeing redesigned in the future release.