Trp-cage - smallest protein
You can find the structure file and images in the PDB database (www.pdb.org) with PDB ID = 1L2Y. This highly stable mini-protein is important for studies of protein stability, protein folding, and 3D structure.
Even with this small size, it displays secondary structural elements, such as an alpha helix, found in many proteins. So far there are no known proteins with less than 20 residues, but we'll see what happens in the future.