European Rules
The European rule  UNI EN 13374-2004 Temporary Edge Protection System fixes the requisites and the testing methods for temporary guards during the construction or maintenance of buildings or other structures.

According to the the stresses reported in the figure above (par. 6.3.1 of the Rule EN13374 and some criteria reported below, this rule identifies the following use classes:

CLASS AFlat or slope roof
CLASS BWith slope < 30° e 60° with fall height < 2 metres
CLASS CWith slope < 45° and 60° with fall height < 5 mètres
(source UNI EN13374 - Enclosure B)

Dimensions (and tolerance) for guards of Class A (paragraph 5.2.1 of the EN13374).