[Moderator's note: I edited your post to use the word "select" instead of "highlight" since I am confident that is what you meant. I also specified that what you want to do is size rather than just select the text. If I was mistaken, please change it back. Feel free to lambast me.]

Yes. It is possible. Anyone trying to help you with this is going to want to know how you are preparing the labels.

Mail Merge? (This is most efficient.)

How many labels at a time?

Can you provide a sample document? How to attach a screenshot or file in this forum. (Word file, not screenshot)

How familiar are you with writing macros in the vba editor?