Icon export @x ratio from iPhone 5 to 6+?
Ricardo is correct in his answer of @3x, but I'll take a shot at answering your question as well.

If you're working from Retina resolution assets already, the upscale factor would be an additional 1.5x for the iPhone 6 Plus.

For example, your 88px by 88px Retina icon would have to be up-scaled to a resolution one and a half times that amount for the iPhone Six Plus: 88*1.5=132 .