The basic installation of Windows 8 or 8.1 needs 16GB for 32-bit version and 20GB for 64-bit version. Click here to see the system requirements for Windows 8.1 installation.
Though it mentions 20GB for Windows 8.1 64-bit version, it will easily reach 25 to 30gb with a few basic software installations and other options. We can not do much with a basic installation of Windows, we do need some additional software installations for our use, don't we?
Say for example, if you install MS office, it consumes some space depending on what options you select. If you enable hibernation, it consumes space equal to your RAM, i.e., if you hav e4GB ram, it needs 4GB HDD space for hibernation. Regular software updates also take up some space which will eventually add up.
So though only 20GB is required for installation, it is safe to assume that it will reach 30GB sooner than you expect with just a few software additions.
So while creating harddisk partitions, it is a good idea to have atleast 50 to 60GB for OS partition. Installations can be done to other partitions, but it is always good to have some free space in the OS partition.
In case you are considering a 32GB tablet with Windows installed, I suggest you to stay away from it unless there are serious budget limitations. You will hardly have any real usable space left in a 32gb tablet. Go for a 64GB tablet. In 32GB tablet, you will encounter low diskspace warning very soon. You can install applications/software to removable SD card, but then the apps will not be availble if you remove/swap the card. So it is always good to have installations on the internal drive on a tablet.