Changing the text message font size on your samsung galaxy s6 is different than changing the font size on your entire phone settings. To change the text font size open your text message section and click the top right tab that says more. Once you do that click font sizes and from in here you can adjust the text message font size on your galaxy s6.
There are seven different choices available to choose from. If your someone who has a tough time seeing than the medium or large fonts would be your best option. If you want to go a little smaller id reccomend doing the tiny size. There is one main benefit to going smaller, and that would be once you adjust the font size to tiny you can fit more onto the screen which lets you see more messages without having to scroll down the screen. Whichever font size you choose for your galaxy s6 there is a wide variety to choose from.