2.1.871 Part 4 Section 3.14.11, oleLink (OLE Link)

a.   The standard does not specify a limit for the @progId string.

Office limits the @progId string to a minimum of 1 and a maximum of 255.

b.   The standard indicates that @progId has a maximum length of 255 according to ST_Xstring255.

In Office, @progId and the referenced target in the corresponding relationships part have a maximum combined length of 254.