200 x 200 inches is Distiller’s limit for PostScrip, it’s based on the Windows printer driver limitation.
The maximum page size is 15 million inches, this is from the PDF Reference 1.6:
In PDF versions earlier than PDF 1.6, the size of the default user space unit is fixed at 172 inch. In Acrobat viewers earlier than version 4.0, the minimum allowed page size is 72 by 72 units in default user space (1 by 1 inch); the maximum is 3240 by 3240 units (45 by 45 inches). In Acrobat versions 5.0 and later, the minimum allowed page size is 3 by 3 units (approximately 0.04 by 0.04 inch); the maximum is 14,400 by 14,400 units (200 by 200 inches).
Beginning with PDF 1.6, the size of the default user space unit may be set with the UserUnit entry of the page dictionary. Acrobat 7.0 supports a maximum UserUnit value of 75,000, which gives a maximum page dimension of 15,000,000 inches (14,400 * 75,000 * 172). The minimum UserUnit value is 1.0 (the default).
15,000,000 inches = 236,74 miles = 381 kilometers