I did quite a bit of searching on this topic, including reviewing in detail the two topics StackExchange immediately suggested for me (How to change text style in qgis2threejs?, Changing font in QGIS2threejs) but have not found an answer. The second of those links states "In QGIS2threejs, I want to change the fonts in 3D presentation. I learned that I can do it by change it in the css file" but the screen shots s/he provided for editting QGIS2threejs.css don't match the current version of that file (the post is over 4 years old). I tried all sorts of edits to the CSS file but with no luck.
The other post suggests editing the HTML file that QGIS2threejs generates once you have your model set up. For example, I added to the HTML file
<style>
.label {
font-size: xx-small;
}
</style>
But that didn't help anything.
The problem I am trying to solve is making these labels smaller:
I have selected "small" in the Label dialogue box setting up the model.
Any Suggestions?