You can set an approximate movie length when initially creating your movie with Magisto. Magisto does initially set a default length based on the footage uploaded, but you can always change it to your liking. Note that you may need to upload a certain amount of footage to be able to create movies of longer length.
After following the initial steps to create your movie you will see the option to "adjust settings" before you select "make my movie". Here you will see the movie length setting and you would just drag the slider to adjust the length as you see fit.