would be great but unfortunately the limitations of region sizes are a majority part in financing SL as a whole, it's the same reason the price to rent a dollar's worth of virtual dirt still runs in the hundreds of dollars a month as you're not really paying for the hardware but rather the exclusivity of the software itself.

i'd suggest checking out the wasteland regions, they've put teleporters all along the borders of their regions so that you're automatically teleported to a higher/lower layer when walking off the edge, and it makes for an enormous environment by SL standards.

Arduenn is correct. Sim sizes are not something that can be adjusted. However, if LL had an option for ultra low usage sims similar to old void sims was available, it would allow owners to do what you're describing. That would be a LOT of work on LL's end though.

Land prices are related to surface area, not to volume, so I don't think LL is going to go along with that. But one can always hope they'll prove me wrong.