If the 2 and the 8 are the only drives you have, and you asked for a 2-way mirror, your 2-way mirror will only have 2TB capacity. I guess the remainder of the 8TB could be used for non-redundant storage if you like to use that.
The interesting case is if you have for example a 2, 5 and 8 TB drive, and ask for a mirror: You will get 7 TB capacity. And if you replace the 2 with a new 12 TB, your usable mirrored capacity will go up to 12.5 TB, using every single byte of the disks.
With wider layouts (in particular the ones that are commonly used with RAID-Z and -Z2/-Z3), the combinatorics gets tricky, and figuring out the capacity on the back of an envelope may be too difficult.