There is no minmal or user customizable Knoppix install. There is only one type of Knoppix install and it is full. You will need at least 2.2GB, or 3GB total (root partition + swap) of space to be safe. Once it is installed you can easily reduce the amount of space it takes up by remove programs that you don't need or want.
If I am wrong, I'm sure someone will correct me.
I hope this helps.
James