w = ExampleData[{"Text", "AeneidLatin"}, "Words"]; n = Length[w]; R :=
  RandomInteger[n - 1] + 1;
P[L_] := Module[{s = ""}, Do[s = StringJoin[s, " ", w[[R]]], {k, L}]; 
  s]; Manipulate[P[L], {L, 5, 40}]