Notices

You need a SSH-server to be installed on the device. You can then enable SSH mode, SSH in from your computer via USB-networking, and edit things. And yes, /home/ is unmounted at that time(same as the rootFS), and can be safely edited over USB.

If you just want to change the size, run backupmenu, connect to your PC(preferably a Linux one), hit 'W' for writable-USB Mass Storage mode, then use your chosen program to edit the N900's eMMC from the computer(as it will be exposed in it's entirety).