My preferred choice what type of railway should be is a standard gauge short line, or branch of a major railway. That way you can run small locos, steam or diesel, and almost any type of freight
car can appear on the layout. In addition, it will all look reasonable.
Many people who start out with a large layout as a first project find out that it's a much larger project than they thought, and never finish it. (Yes, I know some people say, "A model railway is
never finished", but take that to mean "There's always room for improvements" instead. Even if you have, all track put down and operating well, you can always upgrade your scenery, rolling stock,
buildings, and so on.) Always having a half-finished layout isn't much fun.
A good way of measuring layout "size" is counting the number of turnouts on it. Too many and it's too complex, too few and it lacks in operating potential. As a beginner, you should aim for between 5
and 10 turnouts, money, and space being the deciding factors.
Design a compact layout that will fit the space that you have and can be expanded to your dream layout. You will find out that starting with a small compact designed layout will be overall fulfilling
and enjoyable. Designing a layout that can be expanded is easy, just have a turnout or two going to nowhere for now. Later when you are ready to expand, you just add track to the turnouts and have
your main line now become the branch line and start on the new mainline.
Unless you model some kind of self-contained industrial operation, your trains need somewhere to go. I think you need a fiddle or staging yard. However, I'm not sure it really has to be hidden: An
exchange track can be on the layout and have its cars exchanged between operating sessions.
In addition, unless the whole layout has to be permanent, a fiddle/staging/shadow yard turned 90 degrees to a shelf layout is usually enough "out of vision" to work as a good fiddle yard. In order to
save space, it should be removable and only in place during actual operation. If you want it as part of the scenery, something like a ferry terminal would be a good idea.