MusicTechHelpGuy drops a pattern loop into Logic Pro and shows how to fit it to an eight bar section. Pattern loops behave a bit differently from audio loops since they can carry region chords, which feed into the chord track for shaping harmony.
To shorten the loop, use the trim tool on the edge of the region, or set the playhead where you want the cut and press Command+T to split. Either method gets you a clean section length to work with.
Once trimmed, select the shortened region and hit Command+R to repeat it, quickly filling out the rest of your arrangement.