No worries, it's a very relevant question!
You would have to set it up manually choosing the "Something else" option in the installer. It's essentially repeating the same steps in this post:
In step 6, I create a partition dedicated to user files, specifying the mount point as /home. The steps would be exactly the same but the mount point for logs would be /var/log.
How much space to allocate to that... well, I guess it depends? In my current system /var/log is only taking 25 MB. But if the whole point of doing this is to allow for some extra room in case things go wrong, I would be a little more generous and give it at least 512 MB or even 1024 MB (1 GB).