On a different page of that same website, I found the (possibly outdated) info that BSP makes a default viscut every 1024 units:
Pretty sure the last large grid test I did showed it is still the case. (The height or z-axis of 8192 causes major problems as the engine tends to see over everything in a large open area, unless vis-blocked.)

From: http://www.slackiller.com/tommy14/rspeeds.htm
"BSP initially divides your map into 1024x1024x8192 sections."