There is a small mistake. In fact, <dtml-let Size='big'> is interpreted as " Size equals the value of the variable named 'big' ", not the variable named big. And <dtml-let Size="big"> is interpreted exactly as <dtml-let Size=big>, while <dtml-let Size='"big"'> simply doesn't work.

hah, it is rather tricky.

I stand corrected =) That'll teach me not to test things.