In what unit? Inches, centimeters, feet?
When you make a new file you can enter your unit of measurement in the top right next to the name:
Or you can use presets of regular paper sizes like A4 etc. here:
Since you probably know the size of a regular paper you can base it of that, too.
If you already completed a drawing and looking to see how big it already is you go into edit > change canvas size, and you can switch the units on the right to what you need to see, and it'll show you!
