If something has a colored tag it generally means that it is not normal dialogue, but special text placed by some kind of plugin or script.
Those can be very different from one another.
Usually there is a value somewhere that determines the font size for everything, for example at the start of the System.json in MZ.
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In the plugins.js for MV (if they use the standard plugin.)
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or using a Font.default_size N line in the Main section of the Scripts.rvdata(2) for VX and VX Ace.
But there should also be individual values for the different parts of the game inside the files.
For example let's say we have an MV game using a small window to log on screen what is currently happening and we want to change the font size of that log.
Then you open the plugins.js in www>js and you search for a plugin that sounds like something that handles logs, and "usually" you will find a font size option there.
For example in this specific case it was the "TMLogWindow" plugin handling "addlog" entries.
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If you can't find it there it might also have a static font size value in it's plugin file inside the plugins folder.
For VX Ace those values will most likely be in the specific Scripts section of the element you are trying to edit, which means you have to open the game in the official editor and look for the needle in haystack there. It's really tedious which is why I generally just use the Main catchall and make font for ants.