I am assuming you are talking N scale. I am new at this, but it came down to wants desires and needs.
Some of those were being able to pull off the mains and onto the siding and rest at a station. Keeping the mains open. But in doing so, those turnouts take up needed space. By at least one car. So having an Arrival / Departure area was fine, since my layout is larger than most.
Also I wanted to have a yard lead long enough to help / cheat and have even longer lines of liners.
And I wanted to be able to pull of the mains and go straight to the Yard lead.
I also wanted a train/liner to be able to pull off to the A/D track and then have access to pick up some freight also, then pull out again. Or to have a train pull in.. detach for the arrival.. and hook up to another set of liners.. and take off again.
Now thats not to say I got all this.. but in some cases I had to settle for one side.
I was warned by a friend saying the shorter the turnouts.. the more problems you might have with liners going over short turnouts. So.. I went and bought the longest turnouts as possible.
Then again take into consideration the turnouts can cause a derailment... so I placed re-railers here and there.. after/before the turnouts to aid in keep the liners on track. Then again.. the re-railers should have some function other than that in terms of having actual roads where the turnouts are.
But in short.. to answer your question... I would say.. no mater what.. if N Scale.. having a nice station and a yard.. you would need a minimum of four feet. I guess it also depends on how long you want to run the liners. But just guessing.. even with a four feet designated area.. a minimum of four feet is what I had, and barely fitted in just about everything I wanted.
Now of course I am new at this, but I've read so much its been overwhelming in so many ways. I feel humbled. Below is a pic of trains/liners of my N scale layout.
One diesel and four liners plus two re-railers and two turnouts is about four feet long and then some. Now on the other main, I do have a very long yard lead, just in case I want to see how long a string of cars I can pull. Should I find something very useful, I'll come back and add to this. I hope this has helped.