A layout of this size is a monumental task to complete. It is a project of a lifetime, literally!
Derek:
I think everyone here is giving you good advice. Here's some from a guy who presently has a layout under construction.
When I was a boy, like many other boys, I started with an L-shaped layout comprised of two 4x8 sheets of plywood. I was thrilled when I was allowed to expand to three 4x8 sheets arranged in a 12' x 8' rectangle. It meant I could deploy O72 curves. It's probably fair to say that I worked 2-3 hours a day on that layout from ages 13-18. My dad dismantled it for me when I was in my late 20s so he could reclaim the space. He marveled at how much work had gone into it all. "All those soldered connections!" was among his other observations.
I was fortunate enough to move into a house about 10 years ago that has a full-height third floor. The 3rd floor has the same footprint as the house itself. I took down two non-support walls and ended up with layout tables that are approximately 36 x 24'. This is an open dog bone-shaped layout that wraps around a 4'x4' chimney, so it's not one big rectangle.
Big Boy's comment that I quoted above is spot on. One thing that you tend to minimize in your mind when doing something like this is just how much time building a layout takes. Oh, sure, it only takes a few hours to saw some boards and put up some benchwork, but as an adult, your time turns out to be the scarcest of scarce commodities. I know you have two young sons. This means you'll soon be getting to the point where you'll be spending your Saturdays at soccer games, little league games, or even just playing catch in the back yard. You'll find yourself on the sidelines of more than one athletic event applauding your sons' hustle or playmaking skills and -- it's hard to believe -- not even thinking about trains.
About 10 years down the road, at least one of your children will propose that some of the space you've got be re-purposed for his particular passion, if it dares not to be trains. You might also consider that you and your wife's needs/tastes might change over the next 10-15 years, and you might consider moving to a different house/neighborhood. Whatever building you build today will need to be salable to the next guy, our you'll risk diminishing the marketability of your property.
I guess my point is, perhaps think about this in two parts. Part one would be to build whatever layout you think you can build with the time you have available now. Think in terms of the next 10-15 years. If your goal is to have trains running for your sons, keep it small and focused because you won't get it completed otherwise. Think along the lines of the lovely work that Christopher Esposito does. (He reliably posts pictures of it almost every week in the "Weekend Photo Fun" threads.)
Part two would be to think about the time period during or after your sons are in high school. By that point you'll know whether they're interested enough in trains to spend as much time on them as you do. It might be quite a good teenage boy project to work together to build a mammoth layout. Then you could continue to work on it after they're starting their lives when you'll find that you have much more time available.
One final factor you should consider is cost. It should be obvious -- but somehow the size of a grand layout covers one's eyes with stardust. A layout that's 3-4 times the size of a simple 8x8 rectangle will actually cost 3-4 times as much to build. I.e., Surprise! There are no economies of scale! The wood for the benchwork is relatively reasonable but, as just one example, I calculated how much copper wire I'd need to build my large layout, and I found that I was staring at a bill for $700. And $3,300 for track. I'm building with tubular track and the cost of Moondog rubber ties -- which I got a pretty good deal on -- also set me back $750 or so for the 5,000 that I "needed". I could go on, but I'm sure you get the point. And all of what I described is just to place the track on the table and wire it. Those costs are before I get to buildings, accessories, etc. Those Korber kits at $80 a throw add up...
So, my advice would be to first consider what your life offers you in terms of the time you currently have available, the time you'd like to spend with your children, and what your mid- to long-term plans might be for your home. If you keep it relatively simple now, you can involve the kids every step of the way and then tear it down later and really go for it with them as active participants.
HTH.
Steven J. Serenska