To determine the size of the disk, I would recommend something like expected write-load in bytes per second and multiply this number by something that makes you comfortable (I would say at least 6 hours).
Basically, if you lose a machine, the log needs to be able to keep all writes to this lost machine on disk. You also should alert if this disk starts running full.
But your config looks wrong to me (or it looks like it doesn’t do what you want it to do):
This means that each process can use up to 13GiB of memory - but your machine only has 16GiB? I think what you want is that all processes together are not allowed to use more than 13GiB of memory? If that is the case you need to divide this number by number of processes.