Thanks for the interest! My hunch is that 1Chart in its current form would be usable on an iPhone 6+, but the catch is that if we open the door to the 6+, we open the door for older/smaller iPhones, too. (It's just the way the App Store works.) "1Chart for iPad" would probably be a challenge to use on a small 3.5-inch screen.
If I could avoid disappointing users with smaller phones, I would definitely look into the 6+ option. For now, the iPad Mini is the most compact option.
All of the above applies to 1Chart in its current form. An iPhone-optimized version is a possibility.