Step #4 partitioning problem on 8.04 install

[jmwood52]

At step #4, the default choice is "Guided re-size" and shows 212GB available for Ubuntu. When I click forward, a message box appears with "before you can select a new partition size, any previous changes have to be written to disk" This is where my understanding stops. If I click continue another box comes up with "too small size". What "previous changes" are being referenced and how to I write them to disk?

thanks.