A newer alternative to the Font size setting, the Display size setting allows you to adjust the size of not just the text but all surrounding controls and visual elements such as icons, menu items, buttons and other tools.
How it works
This setting works by effectively manipulating the existing mechanism within Android to size screen elements according to the size of the screen and the screen resolution, by applying a linear adjustment to the final figure. As a result, reducing the display size increases the effective real estate on the screen, having an equivalent effect to viewing on a larger screen size. Increasing the display size reduces the effective real estate on the screen, equivalent to viewing on a smaller screen size.
Due to the way it's implemented, this tends to work in virtually all apps and is less likely to cause strange display issues than adjusting Font size alone since everything is scaled equally. Nonetheless, the possibility exists for this feature to be ineffective in certain apps if they calculate the sizes of screen elements differently.