Now that's a loaded question. There are so many different things to consider which I'm sure others will elaborate on but let me sling a few thoughts your way first. I'm not sure if you have mentioned elsewhere what scale your in but for some reason I'm guessing HO. If this is your first layout, take it slow and easy. The last thing you want to do is jump into something overwhelming and burn yourself out! BUT, at the same time, dedicate as much space as you can to this for future growth. If you get into this and you really love it and you have thought ahead for more space, you'll never regret it. As a somewhat experienced model railroader
I would take over the whole house if my family would let me! Take into consideration things such as the size of engines you will be running and the minimum radius you will need to support those engines as well as realistic operations. Make sure your benchwork is well thought out so that you don't have areas on your layout you can not reach. If you look at the layouts in my signature, you will find everything from 2'x2' to 10'x10' in N scale.