In comparison, iOS 7 took about 200 mb more than iOS 6. So if iOS 7 takes about 3.2 gb, then after downloading iOS 8, the system may take 3.4 gb or so.
Your phone will usually need about 6 gb of free storage just for the update. It takes space to load it in. But afterwards, you get the space back. So, do what you need to do to have 6 gb of storage before installing iOS 8.