Check out this helpful article which explains how to fit a halter to your alpaca and why a proper fit is so important.
http://www.camelidynamics.com/wp-content/uploads/2013/11/HowtoProperlyFitaCamelidHaltersmfile.pdf
http://www.camelidynamics.com/wp-content/uploads/2013/11/HowtoProperlyFitaCamelidHaltersmfile.pdf