Make this character bold, please.
When I got on the elevator in my apartment, the bold, red message appeared saying "Earthquake! Get out of this elevator right now," on the electric bulletin board so I got really scared.
Let’s put this in bold to add more emphasis.
What do you think a fat letter is? If I were to guess, I'd say a bold letter.
The reading for this one is a bit odd. The 太 uses the ふと from ふとい (kun'yomi) and the 字 is just the normal on'yomi reading for "letter." You have to remember this weird mismatched reading pair on your own, I'm afraid. Perhaps if you think about how fat letters don't really fit with the rest of the text, kind of like how the readings of both kanji also don't fit in it will help?