I don't think you can. Even if I code the font to be 20px, then the user uses the "+" or "-" font size option in the browser, it still makes it larger or smaller.
These options are there to assist users. Think of the visually impaired. They often need to boost up the text size.
Personally, I build with pixel size. Then, on a broser with default settings it appears as I want it to. Then if they choose to up the size, that is out of my control.
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These are endless galaxies which are yours.
You can journey to infinity
through the endless passages of the cosmos.
Even better. This all belongs to you.
This is your mind.