My philosphy about "size"
After building a large layout and noticing what takes the money and time in the physical plant, I have come to some conclusions.
One of the biggest money and time sinks on the physical plant is the trackwork, and of the trackwork, it's turnouts. They cost the most, take the most time to build if you lay your own, and they are the hardest trackwork to detail, weather, ballast, wire, and maintain.
Next, there's scenery. Basic brown plaster scenery is pretty cheap ($5 for 25 lbs of plaster, and another $10 for pigments to make my own "dirt" [zip texturing]). Throw in a few dollars for glue and a bit of ground foam and I can cover a lot of layout with some basic scenery. However, moving on to the details - bushes, trees, static grass, structures, bridges, and miscellaneous details (automobiles, people, various other small details) - they add up quickly in time and money costs and you can keep adding more almost indefinitely. Narrow benchwork will cut the time and costs of this area.
Then we come to my least favorite part of the hobby - wiring. Yes, I can do it, but it's extremely tedious and the visual difference between a well wired layout and a poorly wired one is almost nil - yet you will pay dearly on a poorly wired layout when you have electrical issues. Wiring per-se isn't that costly, it's cheaper than trackwork is - except for turnout frog wiring, but I lump that into the trackwork bucket, not the wiring bucket.
In the wiring area, there's also the cost of a DCC system - and the biggest determiner there as to cost is how many throttles you will need at once. You can save quite a bit there if you figure out what system most of the operators you would invite are using and get the same system, then ask them to bring their own throttle when they operate. I know some guys who operate regularly on several local layouts and they have their own throttles they bought for each system they operate on. That's a good way to spread the costs around for a DCC system.
Of course, there's the stuff that goes ON the track, but you don't need a layout to spend a lot of time and money on that part of the hobby, so I am confining these comments to just the layout - the physical plant.
BOTTOM LINE:
Layout cost in terms of time and money is most largely influenced by the number of turnouts you have. Two track plans, one in room A and another in room B that's twice the size - both with the same number of turnouts - will be surprisingly close together in terms of overall cost and time needed to build and maintain.
In short, it's not SIZE as much as it is complexity of your trackwork that influences the resources needed to do a layout. Size does matter too, of course, but not as much as we might think. If you want to reduce the resource needs for a layout, look FIRST at reducing the number of turnouts you have on the layout, then at making the layout smaller. Of course, one often follows the other - by making it smaller you can't put as many turnouts in the plan.
But the reverse is also true, I think. If you want a larger layout, if you don't increase the number of turnouts but you just increase the running length of track, you can enjoy more layout without a huge increase in resources needed. That also assumes you keep the layout shelves very narrow (12-18" is ideal) which then keeps the scenery burden down.