PDF spec itself give bit more context:
In PDF versions earlier than PDF 1.6, the size of the default user space
unit is fixed at 1 ⁄ 72 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 (ap-
proximately 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 dimen-
sion of 15,000,000 inches (14,400 * 75,000 * 1 ⁄ 72). The minimum
UserUnit value is 1.0 (the default).