There is no limit for disk size, when they are bigger then 2 TB you need to use gpt type of partition table when installer ask you.
Best is to have a disk with no partitions on it but if there is only one partition it is no problem the installer will remove it.
For installation you need to put only one disk in slot 1, the one on most lefthand side seen from the back of the 2bignas.
The second dusk can be added later after installation