installing CoreOS using usb device and CoreOS ISO

I have some doubts about installing CoreOS using the image ISO.

I follow the steps in CoreOS docs, in the Install section:

cfdisk -z /dev/sda
touch "/usr.squashfs (deleted)"     # work around a bug in mkfs.btrfs 3.12
mkfs.btrfs -L ROOT /dev/sda1

And then when I run the install script:

./coreos-install -d /dev/sda -c cloud-config.yml

I get the error about not enought space.

The install script of the CoreOS site posted in GitHub doesn't work for me because it tell me that there isn't enought space on device.

Then I have some doubts about it:

1. The hard disk has 250 Gb of memory.
2. The usb has almost 8 Gb of memory.

Why this error?