The Notes app on iOS 9 has received so many new features, including the text formatting option. To change font size in Notes on iPhone devices running iOS 9 or later, and specify a text to be Title, Head or Body do as follows:
1. Open the Notes app on your iPhone
2. Stat typing a note
3. To do some text formatting, tap the Plus icon above the of the keyboard (assuming it is open)
4. Now tap the A icon and as you see, you have the option to specify a text as Title, Heading or Body, as well as making a Bulleted list, Dashed list or Numbered list.
5. Select the text you want to be as the Title of your note, and then touch the Title button in the rich text formatting tools.
6. As you see to change font size in Notes on iPhone, you just need to label the text as Title, Heading or Body.