The iPhone6 Plus uses 1810 and 2915 mah batteries, and the phone comes with the same 5V\/1A, but according to iLounge, the new iPhone can use the iPad charging adapter to achieve faster charging. The iPad charger supports 5V\/2A charging, which also means that the iPhone6plus can have a faster charging speed. Based on preliminary testing with a Kill-A-Watt device and OS X system information, previous iphones could only get 5W of power using either adapter. The 6 Plus can support up to 12W.
You can use 18w, but it doesn't work very well.
The Apple 6plus does not support 18w fast charge, but it can use an 18w charger. It can only achieve 5v1a when charging.
The iPhone 6 Plus has a 5.5-inch screen with a thickness of 7.1 mm [13] and uses a 2.5D curved edge screen, which is curved from the edge of the screen to the bezel, and a raised camera design. [20]
You can't. The iPhone 6 doesn't come with a fast charge. Any iPhone 8 or later will work.