The trick is to go through the entire first step -- "1. Configure Installation" -- then quit without going any further, and save your configuration by answering "yes" when it asks.

(You can save your knoppix-installer configuration under your own filename/location, but if you do, you'll have to reload it manually. So let's not muddy the waters with that.)

The default configuration file is saved as /root/.knofig (note the leading dot -- it's normally hidden). Edit this part of .knofig in your favorite text editor:

Code:
# If set to yes, the program will NOT check if there is enough space to install knoppix on the selected partition(s). Use at your own risk! Useful for example with HD_MAP if you only have a small root partition.
# Possible are: yes|no
# Default value is: no
HD_IGNORECHECK="no"
Change the "no" to "yes" and save the file, and when you restart the knoppix-installer, you will no longer get this error message.

Check out the rest of .knofig to see other options you can play with.