How much space does macOS Sierra take? I have a MacbookAir 11" from 2011, with only 64 GB of disk storage. Originally came with OSX Mountain Lion and now I'm thinking on updating it to macOS Sierra.

My question is: How much space does the macOS take? Will it take most of my storage space only for the OS? Or will I have enough space to work with?
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.