My monerod v0.13.0.2 is getting killed after a couple of hours. I read that I should set --block-sync-size 10, but what does that really do? What if I set it to 1 instead to reduce the memory usage? And what is the default and why was it chosen so high where it would eat up so much memory?
--block-sync-size refers to the batch size of blocks to sync when downloading new blocks. It defaults to 200 blocks. With a synced up node it really has little effect as you are typically only getting/sending a few blocks at a time. However, syncing from scratch, it will of course affect how many blocks are transferred each time. Tweaking this will affect how quickly nodes sync from scratch.
It's unlikely this setting is your problem. Probably best to create an issue on GitHub with as much detail as possible (log file, crash log, version, OS etc).