The issue here is that small enclosures can cause all sorts of problems for tortoises including death. Its a bad deal, and its all too common for people to have tiny enclosures, which includes adult sulcatas stuff in small heated sheds or basements for months over winter every year.
Remember Maggie's awesome tortoise Bob? Died of a bladder stone. I see this all the time in Russians that are housed in those "tortoise houses" that people buy online, or in the 40 gallon tanks that the pet stores sell them in.
It also hampers digestion and causes impaction. Much like a horse, tortoises rely on locomotion to keep food moving through the gut. Put them in a small area, and things start to get backed up.
Lack of muscle tone due to lack of area to roam is another issue. This is a major killer of island giant tortoises in zoos that keep them in tiny enclosures.
The 4x8 foot thing is an arbitrary number that I made up over a decade ago. My thinking at the time was that is was so simple to go buy a sheet of plywood at the hardware store, attach some walls and have a suitable enclosure for an indoor Testudo species. Even then, I said 4x8 should be considered a minimum and bigger would be better, but that number stuck and other people started repeating it. Obviously, this does not mean the tortoise is going to suddenly drop dead of the enclosure is only 94 inches by 46 inches, but there is some point where the enclosure size is getting too small, and it is going to do harm to the tortoise. There are many variable that determine this point. I'll agree that 6x3 feet is better tan a 40 gallon aquarium, and that 6x3 feet might be okay for the long term for some tortoises, like the ones Steve mentioned, but is 6x3 big enough for a big 10 inch active Russian female? I say it isn't. Does it mean instant death if the enclosure is only 2x4 feet? Obviously not, but its also obviously not good to house and adult Testudo in something that size, or an adult sulcata in a 10x10 shed over an entire frozen winter.
In conclusion, I will always agree with the sentiment and the statement that if anyone cannot meet any animal's basic needs, then that person should find a home for it that can. When we say that to people it is usually because they demonstrate an unwillingness or inability to care for a tortoise properly. Tiny enclosure size is just one aspect of this. When someone says, "I can't afford to do this or that...", then they cannot afford to keep the animal in question. Its that simple. I turn down free animals all the time, like horses, because I am not willing to spend the time, money, and effort needed to house and care for them correctly.
