1. Why drag is 1/4 mile?

A quarter-mile [1,320-foot (400 m)] is enough the vehicle to reach the maximum speed. Top Fuel Dragsters and Funny Cars are now running 330 mph (530 km/h) in a quarter-mile [1,320-foot (400 m)] race.

2. Can I have a big turbo to generate high horsepower in the same time having good low end response?

Due to engine and turbocharger have their own limited efficiency range, this is hard for an engine to have good low end response and in the same time having high horsepower at top end. Anyway, engineers keep on improve turbo and engine efficiency range by new technology, this might be happen in the future.