My tanks tend to be sparsely stocked...we won't talk about these dang goldfish...and I have moved heavily towards non-heated tanks in recent years. Heating is by far the most energy-intensive part of aquarium keeping; pumps run for peanuts compared to heaters. Lighting? I use shoplites; cheap to buy, cheap to run, especially in the new-fangled age of LED's.
I don't feel the need for my tanks to look like industrial washing machines on the Heavy cycle, and frankly wouldn't hesitate to run any size tank using strictly air-powered sponge filters and/or side-mounted sumps operated by air-powered dual-siphon setups. I well remember my first actual "fish room" many years ago and the fact that there were no water pumps in it aside from the utility pump I used for water changes back then. I still think a large central air pump combined with a multi-outlet manifold is likely the single most useful fishroom improvement one can make.
I don't have issues with humidity; all tanks are covered, and dehumidifiers combine with ventilation to keep my air around 40% humidity. In fact, in winter, I actually uncover some tanks to raise the humidity above the Saharan levels that wood stove or electric heat produce.
In fact, back when I had an Artesian spring running into my basement, I could have handily maintained my tanks with no filtration whatsoever; a constant flow-through system is the ultimate in water quality management. Filters are just a feeble attempt at mimicking the effects of continuous new fresh clean water.
For me, the limiting factors are:
1. Space. I have retired into a little bungalow with a half-basement, so I have only so much room for any of my hobbies.
2. Time. I don't want to turn my hobby into an unpleasant chore; done it before, not going back there.
3. Water! My well seems to be more-or-less bottomless, but I don't want to test its limits too much. I change roughly 900-1000 gallons of water weekly, year-round. No buckets, this is all done by opening and closing valves and turning pumps on and off. I am basically sitting on my butt throughout most of the procedure, just watching.
4. Structural integrity of the house. I don't know how big of a tank could be placed on the poured slab in my basement, and don't want to find out the hard way. Ask an engineer, you say? Sure, go ahead; between covering their own asses, not knowing the details of my home's construction and simply disagreeing on the sum of 2 + 2, you will never get the same answer from any of them.
Overall, I'd say that I'm trending in exactly the opposite direction to that chosen by Esox. He's leaning more towards a few smaller tanks; if I were to start from scratch right now, I would likely go for a single big tank in the 800-1000-gallon range, set it up for minimal maintenance and semi-automated water changes, and call it a day. I'd always have a couple smaller tanks ready for emergencies, hospitals, quarantine, etc. but they would only be set up when needed, using matured sponge filters kept ready and waiting in the big tank's sump.
I like to think that I would still keep fish outdoors in the summer, but sadly those fish need indoor winter homes as well so that means more tanks. In a perfect world, my single outdoor pond would be something like 6 - 8 feet deep, with liquid water beneath even the thickest ice layer in winter (our ice is usually 4 feet or so thick in mid-winter). The inhabitants? A few goldfish and a colony of Rosy Reds would do nicely, keeping mosquitoes down during the warm months.
