It seems you can nest the <small> tags to go diddy. 
Nature’s first green is gold,
Her hardest hue to hold.
Her early leaf’s a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.
Nature’s first green is gold,
<small>Her hardest hue to hold.
<small>Her early leaf’s a flower;
<small>But only so an hour.
<small>Then leaf subsides to leaf.
<small>So Eden sank to grief,
<small>So dawn goes down to day.
<small>Nothing gold can stay.</small></small></small></small></small></small></small>