This is similar to the Pitch shifter. You can change the length of the sample here. Changing the length normally changes the pitch of the sample too, but if you check the box labeled Keep Pitch JAZZ will change the length without changing the pitch.
You may specify the new length in seconds (upper sliders) or in terms of midi speed (lower sliders). With the upper sliders you specify the delta length, that is how much time you like to add or subtract from the original length. Eg to make the sample one second shorter, you would set the first slider to -1.
The midi speed sliders are best explained using an example: Lets assume you have a drum loop recorded at a speed of 120 beats per minute. To change the speed (and thus the length) of the sample, to 130 beats per minute, you set the slider old speed to 120 and the slider new speed to 130.
The meaning of the window slider is described in the Pitch Shifter Dialog.