Bacteria perhaps have smallest chromosomes in organized fashion though they lack nuclear membrane.
Their chromosomes were the first to be synthesized artificially. Artificially constructed chromosome should have the telomeric , centromeric and replication origin sequences . Bacterial artificial chromosomes (BAC) could be used to multiply selected portions of DNA. Yeast are eukaryote or perhaps smallest eukaryotes for which artificial chromosome has been synthesized. Mrs. Okazaki wife of famous Dr Okazaki of Okazaki fragments was still working on Human Artificial chromosome (HAC) when I met her in 2000 in Japan in her lab . Once it will become a routine to synthesize artificial chromosomes the medical treatments will perhaps become easier as the defective chromosome could be replaced by tailor-made healthy chromosomes. However only a doctor could know it better.