Hello,
Grub will not do the partitioning.
Grub is a bootloader to select the different operating systems.
The partitioning is probably done by the install program with the use of a partirtioning program like Gparted.
I always use manual partitioning, when I install linux.
I make a root partition of about 20 Gb, where all programs will be stored.
A swap partition of about 2Gb, this is used by linux as swap space.
And a home partition, wich will be the size of the rest of the disk, this will hold your personal data.
Using this, you can reinstall linux without loosing data.
Bertus
PS I am using OpenSuse ( https://www.opensuse.org/en/ )