Is it possible to:
reformat the WB partition,
then without turning off amiga,
install the workbench disks from floppies.
Other than Windows, AmigaOS *does* allow you to format the disk you booted from. But you won't be able to work afterwards. Better boot from floppy disk.
Put the Install disk in to the floppy drive, then reboot. When the Amiga has finished booting, open the Install disk icon, then the Install drawer and finally select the language you want the installer to run in.
When you are asked for your skill, do
*not* select Novice!
For everything else just follow the on-screen instructions.
And always keep all your floppy disks write protected! If the installer asks you to remove the write protection, then you did something wrong. Do not remove the protection at any time!
And before installing WB, how can I set the size of the WB partition in the HD ?
At first you tell us that you don't even know how to work with floppies and now you want to change partitions? Don't you think you should begin with small steps?
The program to change partitions is called HDToolbox and is in the HDTools drawer on the Install disk. But you really should only use it when you have a little bit more experience. Changing partitions will remove all data on these partitions.