storage spaces - 4 disk mirror size?

I am using storage spaces with the pool consisting of:

disk 1 - 2tb

disk 2 - 2tb

disk 3 - 1tb

disk 4 - 1tb

I created a virtual disk with a two way mirror, the usable space is somthing like 1.82 TB....

I would have through it would be 3 TB? Can you explain how that works?

If I replace the two 1TB disks,with 2TB disks what will the usable space be??

Thanks

Tim