Q: Space required for windows 10

System Requirements:

• An Intel-based Mac
• A Microsoft Windows installation media or disk image (ISO) containing a 64-bit version of Microsoft Windows 8 or later*
• An Apple keyboard, mouse or trackpad, or a USB keyboard and mouse
• A 16 GB or larger USB flash drive that you can erase**
• At least 30 GB of free disk space on your startup drive

You may be able to do this if you free up at least 10GB's more of storage space. Mac OS X requires at minimum of 8GB's.

If you hae the 128GB flash storage MacBook Air, you need to be aware of available storage space.

How to install Windows using Boot Camp - Apple Support

You do not have enough space on the USB to integrate the Windows ISO and BC drivers. You need at least 8GB, 16GB is better. Also, BCA saves the software in /Library/Application\ Support/Bootcamp/WindowsSupport.dmg .

You may want to run Disk Utility and Repair disk, otherwise incorrect disk structures can cause disk space information to be incorrect.

Please use a USB2 flash drive, not a USB3. USB3 will cause problems.