I'm sorry if my title isn't descriptive enough, I run out of space :3
Also this is my first time posting, so I hope I did so in the corrept place. I recently installed KDE Neon, and it's great...
but I have 2 problems, and they're really annoying.

1) Some fonts are too big.
After messing a bit with the font settings I realized my google chrome toolbar(as well as a few others) are... a bit too big. I restored my font settings to
default in order to fix them, but it didn't work...
Here are my settings and chrome for comparison.

2) I can't change my application preference for my files... I mean I can, but after I do it just resets to what it was before...
Eg. I will change a .js file to open with atom(default is kwrite). I then try to run it... and kwrite opens. If I go to change it again I see that kwrite is again the default.

How can I solve these problems?

Man,
If I'm not mistaken, in Chrome itself that changes the font size.

Now about you second question, I don't know, working with infrastructure, Dev's not is my area

Well unfortunatedly, even messing with chrome's font settings won't fix it...

System Settings > Application Style > GNOME Applications Style (GTK)