2.1.626 Part 4 Section 3.3.1.5, cellSmartTagPr (Smart Tag Properties)

a.   The standard does not place any restrictions on the length of the value for the key attribute.

Office requires that the key attribute be less than or equal to 255 characters in length.

b.   The standard does not place any restrictions on the length of the value for the val attribute.

Office requires that the val attribute be less than or equal to 255 characters in length.