Related to this: http://dh.mideal.de/?p=168
After years without progress, here’s a short script for you guys (tbh., it reduces down to a one-liner):
#!/bin/bash
# repair ubiquity’s fixed size windows by Dirk Haar github.2015@github.com
# after booting the LiveUSB execute these commands with sudo
# currently there is only one occurence of set_resizable *with* whitespace
# … but you never know
sed -i -e ’s/set_resizable (False)/set_resizable (1)/‘ /usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py
sed -i -e ’s/set_resizable(False)/set_resizable(1)/‘ /usr/lib/ubiquity/ubiquity/frontend/gtk_ui.py