Based on the level of respect or formality, you can say "How big is it?" in Korean in 3 ways:

• Informal: eolma-na keo? (얼마나 커?)
• Formal: eolma-na keungayo? (얼마나 큰가요?)
• Very formal: eolma-na keumnikka? (얼마나 큽니까?)