I think 8x4" is fine for one adult as long as it gets outdoor time too when weather permits.My redfoot tort is about 9.5 inches and just over 5 lbs, and is about 5 years old. For right now he lives in a 6 foot by 3 foot wooden box when indoors. It is covered in heavy clear plastic shower curtains. I use a heat bulb at each end of the enclosure and one uvb tube across the middle. I use long fibred sphagnum moss as substrate. Here's photo but keep in mind this is in my basement so I don't mind that it isn't beautiful! I personally don't think you have to go 8 x 8. In this 6 x 3 I'm able to regulate temps and humidity to provide a good environment for him. 8 x 4 enclosure would be better, but he's happy in this 6 x 3. I'll be working on an 8 x 4 probably in the fall.
Most people just don't have the space for an 8x8ft in the house.
So IMO 8x4ft is fine for an adult with outdoor time when ever possible. I do think that we could all build something like Tom's outdoor insulated box in an outdoor enclosure. So when temps are lets say 50f+ your tort can wander between insulated box and the outdoor enclosure regulating its body temp.
Mine still want to go out when it's 55f for a wander. When they decide to bunk up and don't retreat to warmth I put them back.
If you have space for 8x8ft then thats better, but you need to think about heating a larger area up. Nothing stopping you having a heated box within an area within your basement, just have a good thick layer of substrate on the cold floor. So for example if your worried about the cost of heating a big area just stick your 3x6ft in a 8x8ft area that is closed off for safety.