It is a bit difficult to know exactly what you mean, because you are mixing up a few different concepts, I think.
First of all, in your title: the \ symbol is not used in Linux anywhere. That is a symbol out of the Windows world.
If, however, you mean / , that is the symbol in the Linux file system for the root directory. You can't prevent files being stored in there, because that is the base of the file system tree, and that is where the system files all go.
You have to simpy make sure that it is big enough. I think 60 GB should be enough, but I don't know off hand how much space a current Ubuntu distribution needs
You can install you /home folder on its own partition. That is where all you personal stuff gets stored by default. If you put that on a separate partition, you stuff doesn't take up room in the partition that the system is on. A further advantage is, if you have to re-install, you can generally just re-mount the old /home partition to the new install, and all your files are just there when the install is finished.