Well to start off with, you've a fatastic space!
Is it an attic, shed, garage, room?
The key thing for me was what I 'enjoyed' was the first consideration.
Do you like
- watching trains go by, or running through countryside
- shunting wagons and putting together trains
- admiring your locos in a depot / tmd style setting
There are lots of ways to build a layout, end to end, fiddle to station, TMDs are hugely popular these days too. The key thing is to understand what you want from the layout.
For me, I enjoy the running through countryside bit, but was more interested in the operational side, so I designed a track plan that gives me lots of operations, shunting coaching stock, shunting fraight, loco release, engines running off shed, per way work, etc.
This way I have a double mainline loop, a branch to a fiddle yard with a reverse loop and three (if not four) busy shunting 'zones' - a largish terminus station, a small DMU stabling and fueling area, a freight yard with per way sidings, a loco shed and fueling point, a preserved steam/diesel workshop/depot and a small cement works / quarry scene.
This way even three or four operators could be busy preparing trains and shunting stuff around while 3-4 trains circuit the mainline giving plenty to watch going buy, or you can just leave the stock looking pretty and watch the trains go by (for me, this was a key requirement in creating a loop / oval / circuit design, so I could set trains off and chill out with a beer )
Let us know more details about the space, the room the layout will be in, if it needs to be mobile, or can be fixed, and we can offer more suggestions