How to take full-resolution screenshots on iPhone? (Getting reduced from 1242 x 2208 to 674 x 1200)

I've downloaded an app (that I built - I am preparing to submit) on my iPhone 6s Plus and I'm trying to prepare some screenshots for the App Store. Per documentation, the screenshots should be 1242 x 2208px, however when I take a screenshot and open it up on my desktop, I'm left with 674 x 1200px. What can I do to get the proper resolution for these screenshots?

Screenshots should be in full resolution, but the way you send it to yourself may cause the resolution to be reduced. In particular, Google Inbox reduces the resolution of photos by default when emailing them.

You should be okay sending it to yourself using Apple Mail, and selecting full resolution.