Here is a link to some layouts that will give you some ideas about size and terrain - alot will have detailed track plans as well.GLI said:
If you are just getting back into it, what I would suggest is to create a mini layout first - a module that is 1x4, 2x6, etc. With a couple switches and some track you can get something up quickly and build on your skills with planning, structures, scenery. Jumping into a 10x10 layout or bigger can be daunting, and it's hard to estimate costs - they can quickly get big which is discouraging at times. A simple thing I do is add up the number of turnouts on the site and see how much that is going to cost. Sometimes that is enough to make you think twice about building a large layout! Unless money is not an issue
This site has a huge resource of track plans for small or modular (shelf) layouts that you can put together in a couple days. I myself will be likely building something along those lines as I don't have the space for a large HO layout - but maybe a medium sized N scale layout!