I have read so many reports about iOS 8 installation storage requirements that I've lost count, and none of them even answer this very basic question.
“The download is 1.2 Gb… the free space requirement for the install is 5 Gb…” etc... But honestly, who cares? We get the space back after it’s done. It's a one time requirement.
What's ultimately of interest when I install iOS 8 on my iPhone 5 is: how much space will I never get back?