What size is the lead screw
You'll need to do some calculation to figure out how long of a lead screw you need. The best solution would be to mock up the entire printer in CAD so you can visualize how everything fits together. Not only is the coupler going to take up some space, but the nut also takes up some space, and perhaps (due to design constraints) you won't be able to have the nut go up right against the coupler so you'll need some more space. Unfortunately, there isn't a general "just take the length of your Z-axis and add X millimeters"-type formula.