Putting a brace under something in LaTeX

Here's a useful LaTeX command that I learned about recently: \underbrace.

It does what it sounds like it does. It puts a brace under its argument.

I used this a few days ago in the post on the new prime record when I wanted to show that the record prime is written in hexadecimal as a 1 followed by a long string of Fs.

The code that produced is is

1\underbrace{\mbox{FFF \ldots FFF}}_\mbox{{\normalsize 9,308.229 F's}}

The sizing is a little confusing. Without \normalsize the text under the brace would be as large as the text above.