1. Check the wubi’s virtual disk file path
sudo losetup /dev/loop0
2. Increase the disk file’s size
sudo dd if=/dev/zero bs=1MiB of=/path/to/file conv=notrunc oflag=append count=xxx * xxx is the number of MiB you want to add
3. Reread disk file
sudo losetup -c /dev/loop0
4. Finally, resize the filesystem
sudo resize2fs /dev/loop0
http://askubuntu.com/questions/260620/resize-dev-loop0-and-increase-space