I am a founder. I'm planning to write a letter/email to sell one of my website to big company, but the question is: Who should I contact?
The CEO of company, the product manager or just anyone that is working at the company? And how to find out the correct email address in order to contact the right person?
It is exceedingly rare for big companies to respond to unsolicited, incoming email from people that they have never met. You are much better off trying to get an introduction to the company from someone you know.
You may want to start at LinkedIn. One of your business contacts may already know someone at the big company, and LinkedIn will find this. It's also a helpful tool to find the person inside the company who is most likely to be interested in your website.
Do some googling on M&A (mergers and acquisition) - depending on what you are interesting in selling (a single website or a business product line) an M&A representative maybe of use. Otherwise, big companies typically will have a business development department (or at least have the contact name searchable on the internet) so you can go that route.