Trimming Sounds
You can trim the start and end of any sound.  Just move the mouse to the right or left edge of a sound.  The cursor will change into a left-right resize arrow .  Hold the mouse down and drag to resize or trim.  (Resizing can also lead to looping.)

Drag to the left or right to get the desired trim.  Zoom in or use the snap setting to trim with better precision.

You can trim the left or right edge of any sound.  The sound will trim based on the snap settings.  If you cannot trim a sound to a specific length it is probably because snap is on and you need to turn it off.

Trim Silence
You can automatically trim the silence from the start and end of sounds by select them and then clicking Trim Silence from the Sound menu.  (This is currently only available for Audio Clips.)

Setting Exact Length
You can set a sound's exact length by clicking on the Sound Detail Tab and editing the Length's minutes, seconds and milliseconds edit boxes directly.