U don't change the lenght u, generally with Audio when u do u timestrecht it... paste the main clip, make sure it has the right timming and size (bpms)... Now hold alt click on the clip and drag the copy 2 a new location... To speed things up (but make sure yer sample/ Clip had exactly the size of one bar) select the clip now press control and k , choose the number of copys and go 4 it!
let me know if this was what u was after,
Z