Optimal Pack Size and how much space needed for System Default Directory?

Hello restic community! Am reading through the tuning parameter docs to learn and am trying to figure out the optimal command structure to compose for my future regular backups. So, I am wondering now about pack size and the System Default Directory if my system might run into some issues space wise.

This is my ubuntu mate desktop setup.

I have an SSD with free space of 673GB (Programs only) and it’s of size 1TB
and a 5TB IDE with 657GB of data

I will want to backup some folders in home, and then from media/ide drive

System Default Directory - size and space needed?
When I launch a full backup of 657GB to a repository on a cloud host like HETZNER, does this mean restic will need to occupy 657GB of data in the system default directory until it is completed? Is this also referred to as cache? It would mean my SSD is not big enough for that operation so this would fail?

Now to pack size.
How would you set the pack size to be optimal? Do I not set it and just use default or what size would you recommend? I have 16GB of RAM.

It says “it is desirable to use larger pack sizes to reduce the number of files in the repository and improve upload performance.”

How high could I set it? Is there a way to gauge/calculate it?