Depending on what other device you have (you've mentioned 4.5") the problem is that many 4.5" devices are running the same 480x800 resulution as the Lumia 530. Which means that their screen real estate is identical, an application cannot differentiate between a 4" and a 4.5" device with the same resultion, thus everything will be larger on the 4.5" device. Unless the application offers some sort of font size setting there's really nothing you can do. Unfortunately, Facebook Messenger doesn't seem to have a setting for font size so in this regard you're pretty much screwed - no way to change the font size, sorry.