Deduce length order of 4 decapeptides based on the Ramachandran plot
When &phi; = &psi; = 180&deg; the peptide is in the fully extended conformation i.e. it is the 'longest' it can be. 

If it isn't possible to deduce a length order based on this information then I guess you will have to do some geometry to calculate the sums of the distances between the &alpha; C atoms using the known values for all of the bond angles - sounds laborious to me!

This site tabulates values of &phi;/&psi; for typical structures (helix, sheet, turn) so it may be possible to decide if some of peptides are adopting these.