How to create and choose sprite sheet for different mobile screen resolution.

Background

We’re working on a 2D mobile game. We want to publish the game on all apple devices which at least support iOS 7 and to all android devices which at least support Android 4 OS. These devices come with different screen size and resolution. For example, in following image we have different Aspect ratios and resolutions that we almost found in every mobile device.

Question

1. Do we need to choose a higher resolution to design a sprite sheet for those devices which share same aspect ratio? For example, iPhone 3Gs and iPhone 4s share aspect ratio of 3:2. But they have different screen resolution.

2. If those devices come with different aspect ratio. Do we need to create multiple sprite sheet for multiple aspect ratio?

3. What are the best practices that unity offer us to design and create sprite sheet for multiple mobile device, which are optimize to even lower mobile device?