After uploading your footage, selecting Editing style and music, Magisto automatically sets the perfect movie length for your movie.
However, you can also Adjust the movie length manually:
After following the initial steps to create your movie, and just before you click "Make my movie" click "adjust settings" - just before you select "make my movie".
On the "Adjust settings" menu, unckech the "Automatic" box and set the desired movie length by dragging the slider. Don't forget to click "Save" to save the adjusted movie length.