quão (not comparable)
- how (to what degree)
quão grande ― how big
- as (to the same extent or degree that)
Ela é tão bonita quão inteligente.
- She is as beautiful as she is smart.
Noun-based phrases are more commonly used, but as it must be a question, quão is mostly used for affirmative phrases.
- qual o tamanho? — how big?