I love web components, but the fact that there's plenty of shadow-dom piercing properties defeats their purpose of "author them once, reuse them in different applications".
One very common pitfall I encounter is the html's own base font size, since it impacts all the calculations in your webcomponents. Use a webcomponent with a font size of 12/14/16 and you get completely different behavior.
If they were truly isolated they would really scale, but they don't.
Shouldn't you be using relative units like rem anyways?
Not to nag, but this seems like a design error? WC font settings should be inherited and relative, rather than any specific pixel size. Designs should be robust enough to support overflowing text, should the user increase scale/zoom/etc.
Admittedly, I might not be understanding your problem well enough, so sorry in advance if I've mischaracterized the issue.