Depending on the resolution of the Windows 10 Mobile phone, the displayed text could be to little … especially if you are over 50 like me!! 😉
Don’t worry, anyway! … you can set font size (and possibly style) as you like that is one of the several setting available in the Easy of Access [IT: Accessibilità] section . You only have to go to Settings -> Easy of Access -> Other options and, in the Visual options [IT: Opzioni visive] put the text size to e.g. 140%.
In the following animated gif, you can see how to do to let the font be sized differently, in order to see better the displayed text (e.g. also the Messages).
However be careful that, if you choose the font size too big (e.g. 200%), it could happen that some SO displayed text will overflow the available space, as you can see in the following screenshots: this easy access feature should be handled better in Windows 10 Mobile and may be next Windows 10 Mobile releases will do that …
Moreover you can also use the Magnifier [IT: Lente di ingrandimento] option that let you magnify the text double tapping the screen and then, holding both fingers, scrolling up or down them in order to set the wanted size: unfortunately with that method the text is magnified only from the centre of the screen so I personally think it is not so useful!