Feels at each thread, and lives along the line.
During the process of making an orb web, the spider will use its own body for measurements.
After strengthening the first thread, the spider will continue to make a Y-shaped netting. The first three radials of the web are now constructed. (the "Y"-thread can be seen in the pic below by her hind legs). More radials are added, making sure that the distance between each radial is small enough to cross. This means that the number of radials in a web directly depends on the size of the spider plus the size of the web.
Camera: Canon EOS 550D - Location: My bathroom court-yard garden, Tarlton, Gauteng, South Africa
(I did post this in 2012, but thought I'd bring it forward)