I am having Lumia 640XL and have installed telegram in it. The phone is running Windows 10. Is there any option available to decrease the font size in Telegram? I have searched for options inside the application and couldn't fine one.
Sorry, currently there is no option to decrease the font size in telegram.
Telegram is not affected by the accessibility settings. It also does not have a built in option to change font size. So you are left to use the default font size.