Keep it larger than 1 millimeter.
Round links are best for hanging pendants.
Flat chains are waiting to break. The safest lighter weight chains are those with multiple strands that are totally flexible. Chains that have a definite inability to flex or bend in any direction have a tendency to break eventually because of this inflexiblity. The thin ones break quickly and the heavy ones can go for many years with no problems at all. There are some chaines with slightly limited flexiblity such as foxtail which are also long lasting and multi stranded.