Yesterday, Skype released an update for its iOS app to include iOS 8-optimized notifications and other goodies. However, it has no support for the higher-resolution iPhone 6 and iPhone 6 Plus displays. That will change next month, per a Skype spokesperson:
We’re currently working on an update to Skype for iPhone that scales for iPhone 6 and 6 Plus screen sizes, which we plan to release in the next month.
Unfortunately, that sounds like quite a while. Hopefully the updated version isn’t just bigger, but more optimized for the larger 4.7-inch and 5.5-inch panels.