Araneus Diadematus), but because the web and its supports are so large, the dot is vanishingly small.
The right-hand figure shows a 25-cm diameter web supported by a 300 cm (~10 ft) line on the side, a 250 cm (~8.2 ft) line to the ground, and multiple short pieces on the left. In other words, the spider is about 250 body lengths above the ground. To put it in human terms, it would be like a 1.82 m (6 ft) person perched on a very fine net that was 455 m (1,500 ft) above the ground. It's somewhat like Philippe Petit's wire-walk between the two towers of the unfinished World Trade Center towers in 1974, which was just over 400 m (1,300 ft) tall. His walk — and the complicated preparations that it required — were vertiginously and beautifully documented in Man on Wire (and also in a book of the same name).
To return to spider terms, they probably don't get vertigo and don't realize how far they are from the ground or nearby objects. From what I've seen and read, web-building spiders are creatures of vibration and the near-field: they respond to things hitting their web and what's very close-by. So hanging out a few hundred body-lengths from a solid object is no big deal...
The left-hand figure has support lines that are 150 cm (4.92 ft) long. In other words, the spider is over 150 body lengths from the nearest solid object. That's like 1.82 m (6 ft) person on a very fine net 274 m (900 ft) above the ground and with 274 m between you and the nearest object on either side. For reference, the Chrysler Building has a height of 319 m, the Empire State Building 381 m, and the Transamerica Pyramid 260 m.
Howstuffworks has a series of illustrations that demonstration how a spider weaves an orb web.
For more about spiders, check out David Attenborough's "Life in the Undergrowth." It's filled with amazing footage and stories from the world of insects, arachnids, slugs and other invertebrates.
The Man on Wire image is from Wikipedia, which claims that this particular image is covered under the Fair Use provision of copyright law. The photo and line drawing are by the author.