The iMovie video effect "Letterbox" adds black bars to the top and bottom of the video clip, giving it a widescreen look.
iMovie Video Effects - Letterbox
Effect In, Effect Out:To apply the effect to only part of the video clip, move the sliders to the places/times when you want the effect to begin and end. Leave them both set to 0:00 to apply the effect to the entire clip.
Shift: This slider vertically adjusts the video clip, moving it up or down behind the black bars.
Size: This slider controls the size of the letterbox. Move it toward None to decrease the size of the letterbox, or toward Full to make the letterbox cover more of the screen.