It depends on what you're wanting to build. If you want it to be very realistic, you will be very restricted in what you can do but, if you basically just want to see trains go around, then you could cram quite a bit in. My layout is 8x4 and has double track, a four platform terminus, turntable, engine shed and a small goods yard, and looks like something out of a 1950s model railway book of plans (it's actually based on the one on the back of the Hornby Dublo book). No matter how realistic I make the scenery, there's no way that it will look like a real railway, but it's giving me more enjoyment than the fiddle yard to terminus layouts I have tried in the past.