Is the Arc de Triomphe as Big as Buckingham Palace?

No. They are vastly different in scale:

• Arc de Triomphe: ~50 m (164 ft) tall, 45 m (148 ft) wide, 22 m (72 ft) deep.
• Buckingham Palace: A sprawling royal residence with ~77,000 m² (829,000 ft²) floor area, 775 rooms (including 52 royal/guest bedrooms, 19 state rooms, 92 offices, 78 bathrooms).

Buckingham Palace is far larger in both height and floor area.