What is the optimal text size for an iPhone 'News application'?

Question

We are designing a News application for one of our clients, and I would like to get to know what is the optimal text size for Headlines and Article text in an application like this?

We would like to maximize readability for users, because this is the most important feature of a News app.

Do you have any research or experience on this topic?

The app would be something like this:

Answer 1

Unfortunately, there's no one answer.

There are a lot of human factors outside of your control that have a large influence:

• Does your user need corrective lenses, and are they wearing them?
  Anyone not wearing their glasses - or who needs them but doesn't know it yet - will favour a larger font.

• Are they wearing progressive lenses - and are they holding the phone low or high?
  Progressive lenses provide a different focal distance at each height - closer lower down, farther away higher up. But, the most convenient place to hold the phone may not correspond with the focal distance their. Again, a larger font might be desired by this user.

• How well do they read? Letter by letter, word by word, or phrase by phrase?
  Someone who reads letter by letter will usually favour a larger, non-serif font so they can pick up each letter in turn.
  Someone who reads word by word will favour a serif font, as that helps each word to be seen as a single "thing".
  Someone who reads phrase by phrase will favour smaller fonts, so that each eye registration pics up more text, supporting their higher reading speed.

For a good default in your application, pick something close to what other people do, but do provide at least small/medium/large options for your users to select.

Answer 2

We developped a website to create mobile sites and we're using 14px for the content. The typo was 12px at beginning but, we had complaints from users. In a mobile site, you need to stay easily readable.