No "F" Words Please

There's nothing more mortifying than when your three year old yells out "Mommy....I just saw a really FAT person!" Leave it to kids to say it like they see it. So I am now teaching my boys that 'fat' is a BAD word. But my question is, is it any better to hear 'Mommy....I just saw a LARGE/GIANT/BIG/ETC. person!" ?