diff --git a/README.org b/README.org
index 37601c9..6331513 100644
--- a/README.org
+++ b/README.org
@@ -98,9 +98,16 @@ For some hardware the official Guix installation image won't do
 nonfree Linux kernel and nonfree firmware with the following command:
 
 #+begin_src sh
-guix system disk-image /path/to/this/channel/nongnu/system/install.scm
+guix system image --image-size=7.2GiB /path/to/this/channel/nongnu/system/install.scm
 #+end_src
 
+As indicated bellow, you will need to run ~guix pull~ to download the
+Nonguix package descriptions. Some free space on your USB thumbdrive is
+required for this operation to succeed. The ~--image-size~ option allows
+you to specify the size of the image and, as such, to allocate free space
+to it. The given value is purely indicative. It obviously depends on your
+thumbdrive capacity.
+
 Then you can write the generated disk image to a USB thumbdrive with:
 
 #+BEGIN_SRC sh