Issue with installation of FFMPEG and Homebrew due to low HDD size?
On my system the necessary Command Line Tools take up some 303.6MB (I am using macOS Sierra 10.12.3 on a 2012 AirBook). Also, for what it is worth /Applications/XCode.app is only ~6+GB on this same computer.

I don't know that this is a comprehensive storage requirement assessment, but...
~300MB for /Library/Developer/CommandLineTools/
~125MB for /usr/local/Homebrew/
~_60MB for /usr/Cellar/ffmpeg/
~ 500MB Total Storage  

STEP ONE:

To install Command Line Tools without the full Xcode install:

$ xcode-select --install


See also this step-by-step guide.

To uninstall simply delete the folder /Library/Developer/CommandLineTools/

STEP TWO:

To install Homebrew:  

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install)"


When using Homebrew, it's good to run brew commands such as
brew update &amp;
brew doctor
...prior to installations. See here for some brewing tips.

To uninstall Homebrew:

$ /usr/bin/ruby -e "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/uninstall)"



  Download the uninstall script and run ./uninstall --help to view more uninstall options.


STEP THREE:

To install the most basic ffmpeg with Homebrew:

$ brew install ffmpeg


After installing ffmpeg confirm that ffplay, ffprobe and ffserver have successfully installed and are available in your command line by using, for example:

$ ffplay -version


If they are not all available, you may need to use the following brew command:

$ brew unlink ffmpeg &amp;&amp; brew link ffmpeg


Some ffmpeg resources:
FFmpeg documentation: http://ffmpeg.org/documentation.html
An FFmpeg support forum - http://ffmpeg.gusari.org/
"FFmpeg Basics" by Frantisek Korbel 

To uninstall ffmpeg with Homebrew:

$ brew uninstall ffmpeg