Use the same yarn and needles for the swatch and the project.
You can make the sample swatch as big as you want. A larger swatch is more accurate. Make it at least 4 inch.
The size of your finished project is an indication.
The numbers belong to your way of working. Someone else may end up with something different using the same yarn and needles. This depends on how loose or tight you work.
Will you be doing multiple projects with the same yarn and needles? Write down the size of your sample, so you can use it again for your next project.