Selfresizing boxes are a time saver by eliminating the needs for extra keyframing. In this tutorial, I go over the After Effects expression sourceRectattime() which can help you create text boxes that resize with your text. It is the perfect expression when making lower thirds.

You can copy this Expression:
s=thisComp.layer("Name");
x=s.sourceRectAtTime(times.inPoint,true).width;
y=s.sourceRectAtTime(times.inPoint,true).height;
[x,y]

Replace "Text" with the name of your text layer and boom. You're done.

My personal site: https://www.keyframeacademy.com/

Programs used:
Adobe Premiere CC
Adobe After Effects CC 2019
Camtasia 9