Calculate resolution for objects from Sketch iPhone UI

Question

I have made a mockup of my UI for an iPhone app I'm making in Sketch. How do I calculate the resolution for each object in the UI and how do I export each components as a .png or .jpeg .So (for example) in the Sketch UI the is a rectangle, how do I export only that rectangle as an image and how can I get the correct resolution (iPhone 4-6 is 326ppi, 6 Plus is 401ppi)

Answer 1

So, this is why I LOVE Sketch 3, the export tool in the inspector. Check out this video they provided.

They're different ways to export using the inspector:

1. Exporting Art Boards: select an art board and in the right hand panel there'll be an export set up to export in PNG
2. Slices: this one allows you to fully customize what to include in your export, layers and a background, etc
3. Grouped Layers: I believe you can also export grouped layers as well. Same inspector as a slice or art board

I hope the video helps :)