No, writing small doesn't refer to the length of a tweeting moment or the words you may think of during an argument. Combined with thinking big, writing small is about allowing your ideas to shine through.
The words you choose are up to you, but try to expand beyond the ones you regularly pick because they easily make sense. The words can be mile-long medical terms, or basics ones of color, sound, motion, taste and emotional depth. It only matters that you use them in unusual ways when you are stalled and can't get past a roadblock.
Think big in terms of the give and take of words and their meanings, rhythms and reach. Then shrink your focus to just that word and why it resonates for you.