actually cowfile_size is limited to values smaller or equal to 100. That way you have to decide at iso build time what size your filesystems should have.
I have added a feature to allow values bigger than 100: In this case the read only device is linearly assembled with a sparse file (we do not have to care about size on disk as no write go to this file), this device is used for read only device then. After chroot (or when the system has bootet up) you can grow your filesystem to the new size.
Growing the filesystem in initramfs would be possible, but we would have to add userspace utilities (resize2fs, xfs_growfs, ...) there...
The patch is attached. What do you think?