Space for your layout - how much do I need?

First and most important advice: "grab" all the space you can.

The absolute most common mistake is having your curves too tight:

• The more room, the longer the mainline, the more room for a switchyard, and the broader the curves you can use.
• The more room, the longer it takes a train to make a circuit, and the less "toylike" in operation.
• Spend time thinking into the future. Yes, many people recommend that you just lay something down right away.
• Is there ANY time in the real world where doing a little planning is BAD? Resist the temptation, and look into the future. Don't just rush out and lay down a small oval, and then hope to expand on it later.
• It's a lot less fun tearing up an existing layout to move it around.
• Look for places to expand, you always need more room than you have.
• Look for ways to broaden the curves, places for passing sidings. 
• Don't start with a small loop if you don't need to, look at where you will be in years to come, and see how to implement it in phases.
• Don't put the track where you cannot maintain it, avoid places that might have to be torn up, or where there is water drainage places.
• Don't do anything less than 10 foot diameter curves in a new layout.
• Spend the $99 for a track planning software, that lets you "snap together" pieces of track and do "what if" very easily, it's hard for anyone to visualize without some form of drawing.