It depends on the tortoise... by: The Greek Tortoise Guild
Over the years, we have had five different Greek Tortoises. Each has had a surprisingly unique personality with equally unique needs. Our first Greek Tortoise LOVED to climb and managed to climb out of his cage a few times while we were away during the day. He was also full-grown and very strong which created a problem with how to fully escape-proof his home. Our next tortoise was much smaller and more timid. The third Greek Tortoise we kept was extremely shy, didn't like to climb, and wasn't very active in general. While we made sure that we were taking care of him well by supplying correct UV exposure, nutrient intake, and even taking him and a stool sample to the veterinarian he just liked to sleep more than our other tortoises and not tromp around very much.
The point is, some tortoises, even within the same species, need more room than others. Some sources that I have read explain that the cage must be a certain multiple of the tortoise's length. But in my experience, this is not a science. You need to build or buy a home for your tortoise that fits his needs. I suggest making a home that is inexpensive yet atleast as big as he will conceivably need. For a full-grown adult this could be as much as 3 feet by 4 feet or smaller for a baby. Make this out of a sheet of plywood with some rocks or something else to test how much he likes to climb (and, of course, include the other reptile essentials like heat lamps, water, etc.). Observe him a lot and at different times of day if possible. Once you feel that you have a good idea of what his individual needs are you can make or buy a nicer and more permanent home. Or maybe you'll find that this home was the perfect size! Either way, start out with something relatively inexpensive to get to understand your tortoise's needs and then make a long-term home to fit those needs. I would not advise multiplying your tortoise's shell length by 5 and then buying a huge tank for a tortoise that ends-up being a recluse.
If you would like to ask me any more questions on this or any other chelonian-related topics you can do so at TheGreekTortoiseGuild.blogspot.com.