I am trying to create bootable flash drive. How big does it need to be

OS X installers are typically 4 to 6 GB. If you buy an 8 GB thumb drive, you should be OK if all you want on it is the operating system.

Rysz is probably correct but you may want to get a 16GB stick to be on the safe side. You can find them for approx. $4.00.

You can make a bootable USB stick to install using this free program which will do all the work for you.

Bootable USB Flash Drive – Diskmaker X