I'm sure there are lots of people who think, or once thought (like I did), that an acre was a huge area of land. Sure, 43,560 square feet, or 4,840 square yards certainly sounds huge... but that "square" in there is a bit misleading. See, an acre is a measure of area, not length, and I see many garden and mini-farm plans claiming you can be "food self-sufficient" on 1/4 or 1 or 2 acres WITH a house... but if you look really carefully, the dimensions on the plans don't add up.
So what is an acre, and what does it look like?
Well, conventionally, an acre is one furlong by one chain -- 660 feet by 66 feet -- but it can really be any shape and dimension that contains an area of 43,560 sq ft.
The easiest comparable "acre" for most in the US to picture is a football field WITHOUT the end zones. The playing field in American Football is 100 yards x 50 yards, or 5,000 sq yd, which is roughly 1.03 acres. Adding in both 10 yard end zones makes it 6,000 sq yd, nearly 1.24 acres.
Can you imagine building a cabin, growing all your produce, raising all your animals and their food on a football field? Can you imagine it if you also had to produce your own CLEAN water supply, sanitation & waste processing, and heating fuel? Can you imagine it if you couldn't rely on municipal grid power or outside inputs (like building materials, animal feed or fertilizer)?
Let's put into perspective with dimensions that are a little easier to wrap a brain around. An acre that's square would be roughly 208.7 feet on each side, about 2/3 of an average city block, 5-1/2 school buses, or 3-1/2 semi-trucks. Starting to sound a lot smaller?
Here's a sample scale layout for a 1-acre S-S farm, 210' x 210'... assuming a 3-season growing period and mild winter (only 3 cords of wood per winter) and minimal outbuildings:
We kept our cabin's footprint pretty small at 16' x 24', the porches add 6' to either side, so 28' x 24'. BUT for fire safety, pest resistance, and ease of access we also need 5 feet of "barren" path around the perimeter... so that's 38' x 34', 1292 sq ft you're not growing anything -- nearly 3% of your acre.
A private road or driveway averages 12 feet wide, ATV paths average 8 feet and foot paths average 4 feet. You have to be able to get yourself, materials, equipment and animals around your homestead, so a significant portion of that acre is going to be dedicated to non-growing transportation areas. We'll be generous and assume that your homestead will be laid out to maximize transportation with minimal roads and paths, let's say 10%. That's a little over 4356 sq ft you won't be growing or raising anything on.
So, you've essentially eliminated 5648 sq feet, or 13%, of your acre just on the cabin and access. In the remaining 38k sq ft, you put in gardens & fields avg 4,000 sq ft per person (about 1/5 acre for a couple) for all the vegetables and grains for humans and their livestock; a dozen laying hens with a 60 sq ft coop & 200 sq ft yard with broilers in mobile "tractors" on pasture until slaughtered (requires "pasture" to put them on!); a dozen breeding rabbits with a 40 sq ft hutch with fryers in a community pen until slaughtered; 3 sheep with a 240 sq ft shed and....
...oops, you just blew your acre with the sheep because they require 1/4 acre of "pasture" for fresh graze, 750 lbs of hay, and 100 lbs of supplemental feed each per year... and you have to have at least 2 or they get lonely (3 if you want to breed). OK - you can go with 3 goats and 2 feeder hogs rotation-pastured in your woodlots, they don't need grass like sheep do, but add 100 sq ft of garden space per hog for feed -- they literally, eat like pigs LOL. You definitely ain't carrying a cow, not even a small one, stocking rates for cow-calf pair on good pasture year-round is 1:acre.
And you'll need to grow their bedding/litter as well... if you use straw you need to grow grains, if you want woodchips you need some sort of woodlot. Don't forget about feeding your household pets! If Fido and Fluffy eat solely off the homestead, you'll need more chickens & rabbits... or throw in some ducks and geese (you need the pond anyway).
You might be able to shave off some of the personal food space by eliminating/reducing grains, but we're including small livestock feed, minimal irrigation and no commercial fertilizers, so the more common "1k sq ft per person" won't necessarily apply. You'll need more space for green manures (cover crops grown just to replenish the soil) and crop rotations. The livestock will also provide brown manure for your garden, but you don't have a factory-farm's worth of these critters and supply isn't unlimited.
You might also be able to shave off some of the personal food space if you live somewhere with an extended growing season where you can eat a succession of fresh produce for 8-10 months in smaller area and not need to grow everything you need for the whole year all at once and preserve it for the remaining 6-8 months (like we do).
So you're down to about 1/2 acre now, and you still have to consider heating fuel (wood lot for firewood)... some of the personal food yardage may be usuable here if you live somewhere where fruit trees grow well and you have a decent-sized orchard that you coppice for firewood. And you have to keep all the poo-related areas (including fertilized garden) and toxin-related areas (driveways, garden, garage, workshop, etc) at least 100 feet and downhill from your well (which totals 1/4 acre with limited use). And your privy also needs 100 foot perimeter for hygiene and sanitation (another 1/4 acre). Plus, you still have the footprints of all the other homestead structures to consider... garage/workshop/toolshed, garden shed, woodshed, wellhouse, abbatoir/larder, privy, rainwater cistern, pond, solar & wind array... they all add up.
So, in a perfect temperate climate with mild winters and really awesome soil, you might be able to be self-sufficient on 1 acre if you don't eat a lot of grain (of ANY kind) or potatoes, or want to make your own fuels (woodgas, biogas, biodiesel or ethanol), or want dairy or red meat or fish or bacon. But for the rest of us...an acre would be seriously pushing our luck with absolutely no room for expansion if necessary. No margin for error or mishap, and I seriously doubt it would be sustainable (notably, the woodlot and soil fertility).