I currently have two 8TB drives, one main and one parity, that is running out of space. If I get two more drives do I need to stick with 8TB or can I bump up to higher capacity drives?

No data disk can be larger than any parity disk. Since your parity disk is 8TB, you can only use data disks up to 8TB, unless you replace parity with a larger disk first.

You don't HAVE to get 2 more drives unless you want to. As an example, if you get a new 12TB drive, you could remove the 8TB parity, set the new 12TB up in the parity slot, and put the old parity drive in data slot 2. That would give you 16TB of usable space, and if you later got another 12TB, you could just add it to the array.