@Vitto size of the layout has little to do with the power required to run trains. Modern engines draw about an amp. Considering what you have said you want to do, a Z4000 is plenty big. What starts getting into trouble are lots incandescent lights (in passenger cars and accessories), multiple engines in a lash-up and anything else that draws continuous power. The other consideration is how you run power feeds to the track sections. Gauge of wire and length of run makes a difference as the power draw increases. Some prefer 16, 14 gauge or 12 gauge copper solid or stranded and then there are buss or home run/star feeder designs. If you are running DCS pick up a copy of the DCS companion. It will help head off wiring problems and signal strength issues if you follow Barry's instructions. He describes power districts and blocks and recommends home run wiring due to the way DCS provides the control signal. In my little layout, each loop has a separate power block. Each arm of the Z will provide up to 10 amps for 20 total.
For what its worth, I power only switches from the Z. Everything else has separate power supplies. I've stripped out regular incandescent bulbs from all my buildings, so everything is either 4.5 or 12 volt DC LEDs.
