From my tests, Flash’ maximum class path size is 74 characters. The symbol name is 64. The symbol name (linkage ID) will popup an error, the class path does not.
I’ve done some tests, and it seems that 74 is the max; 75, and Flash says it can’t find the class file (yes, FLA’s classpath is set, and so is Flash’ global class path).
Searched in Help and Livedocs, nothing came up.