How do I estimate what my website's bandwidth & size will be?

How do I estimate what my website's bandwidth & size will be?

So i am just starting a new website, i have no idea of what the website size will be until it gets to it, how do I estimate what my website's bandwidth & size will be so that I know to buy the sufficient hosting package for handle it? I don't want to buy to low and then it crashes or goes down, but by the same token, I don't want to order way more than I need because that will cost extra for something and the money could go elsewhere.