The longest human protein is Titin with 34,350 amino acids. The smallest human protein is 44 amino acids but it could be an abortive translation from the 5' UTR of another mRNA. The smallest functional polypeptide is glutathione with only three amino acids.

BTW, proline is technically not even an amino acid much less a polypeptide. Chignolin is a man-made decapeptide so if you synthesize a GG dipeptide, it would be the smallest possible amino acid polymer.