This tip explains how to customize the font settings for the Messages app on a Samsung Galaxy Note 5. By default the Messages app makes use of the same font settings as your smartphone your text messages, but if you find the text to small/large, then you can easily adjusted this setting your liking. The procedure is as follows:
Open the Messages app and then tap on the More menu > Font Size. Toggle Use phone font sizes off and then use the slider button to adjust the default font size (the values range from Tiny to Extra huge). Tap on Save to apply your new settings (applies to the Message app only).
One last tip, you can also use pinch gestures to zoom in/out the Messages app.