-laMort. ... always refer to a care sheet as that is the ideal requirements always suggested and the data is mostly captured by scientists in the field.
That's a bit high for a terrestrial T, it would be better for an arboreal, but if you absolutely must use it, add a few extra inches of substrate to lessen the space from top to floor. I use a 10-gallon tank for my B. smithi ,which is a little large, but with extra substrate she does just fine.
Terrestrial T's in the wild would most likely be in underground burrows or under rocks etc, and they generally do not travel very far from their dens/burrows, so I would think the risk of them falling is very low in the wild. Not to mention we are supposed to keep them safer than they would be in the wild I have read some people who have had their T's suffer ruptured abdomens as well as their t's losing legs. It may not be super common, but it is a risk I would definitely seek to avoid. I agree with those who said that the enclosure is much better suited to an arboreal species. That said, I think it is a beautiful set up
I think that if your spider can stand up and basically touch the top that it would be fine. I keep our rosie in a 10 galleon (which is over kill to a lot of people), but have it very full of substrate, and I keep her 'decorations' fairly smooth/soft so if she were to climb (she isn't much of a climber) there wouldn't be much risk of damage. I just commented on the enclosure that was in the picture...you'd need a pretty huge tarantula in there for it to be able to stand up and almost touch the top in that tank lol.