my experience with Mand...
Im using linux from Mandrake 9, and i have some problems with the default instalation too. This instalation make a partition to root directory( / ) and other to the home dir.
Usually i have problems with PostgreSql, when i tried to make large query request. So, i decide install the partition with the graphic tool and chose all HD space for root directory and no home partition (use the clasical directory tree).
Actualy i use Mandriva 2006 and with these instalation, and active the auto delete /tmp directory every time when i boot, and i have no more disk space problems.