How much space does OS 9 use?

MY iPad has 16 GB and has used all but 2 GB. Wondering if I buy a new phone with iOS 9 if I will have any problems with updates. New phone will have 64 gig.

My old phone could not update iOS 8.4.1 because it stated it required 1.6 GB of storage and I had 8 GB total, so I could never download. Don't want the same problem with getting them in sync.