I have to buy a box for my product before selling it?

It's my first time trying to sell something on ebay so please bear in mind if this is a dumb question.

It's asking me at shipping to write down the dimensions of the product. But idk the dimensions since i'm only going to buy a box and box the product if someone actually buys it, not the other way around. So I put idk for dimensions. But then it's asking me how much the customer will pay for shipping? How am i supposed to know beforehand if idk how much shipping will cost depending on where they live and how big my imaginary box I dont have yet is?

am i just supposed to guess the shipping rate? both for within canada and international?