How much space does macOS Sierra take?
A clean install of macOS Sierra 10.12.3 from a USB Flash Drive takes a little less then 10 GB of space. If you do an in-place upgrade the Install macOS Sierra.app bundle takes a little less then 5 GB but can be deleted if after the upgrade it doesn't self-delete. So, you have plenty of space whichever way you choose to go. (The in-place upgrade method is assuming you have adequate free space  to preform the in-place upgrade.)

As always with an OS Upgrade, make sure at the minimum that your user data is backed up, if not the entire system before performing the upgrade. Time Machine is a good way to maintain backups of the system.