Other than changing the font size for the whole Android device, you can just change message font size on HTC One, It only affect the SMS text message size and the other parts of the phone are unchanged.
To completely change the font size on HTC One , see: How to Change Font Size on HTC One
To change just the font size for text messages, do as follows:
1. Open "Messages" App
2. Tap the three dot icon on the top left and select "Settings"
3. Tap "General"
4. Scroll down to the end and tap "Font Size for Thread Display"
5. Choose the font size: Small, Medium, Large, Extra Large