Today we will talk about unit conversion, which sounds complicated but can aid you greatly if you understand how to use it. Here is a fantastic website for converting studs to lots of other units: http://studs.sariel.pl/
You can use the website above to convert studs into mm, cm, m or inches. This is useful if you are making a life-size model, as you can put in the measurements in cm for example and know how many studs that will be. A useful conversions is 1 stud = 8mm or 4 studs = 32mm which I double checked on a physical brick; it is very accurate.
You can also use it to get precise dimensions for other designs. For example, I am designing a Resistance Troop Transporter to minifigure scale so I first got the width of the ship which is 16.8m (found here: http://starwars.wikia.com/wiki/Resistance_transport)
Next I found out minifigure scale in relation to humans, which is around 1:40 considering the width of a minifigure (found here: https://bricks.stackexchange.com/…/is-there-a-standard-scal…).
Then, on the website, I put 1:40 into the right hand scale and 16.8 metres into the left hand side which told me that the total length should be 52.5 studs (2nd picture). This allowed me to make the dimensions for designing the rest of the MOC.
Hopefully you followed that, if you have any questions please ask in the comments. This should be very useful if you are unsure to what size you should build to, and has helped me immensely since learning about it.