Music Maker is designed to keep the duration flexible. However, you could try to use the blue start and end marker that you can position with your left and right mouse buttons above your timeline in the Music Maker main screen and see whether that works for you. Just use the minus from the horizontal zoom function to zoom out of your arrangement and position the markers as needed. This might come in handy if you actually want to add a certain duration of silence at the end of the arrangement when mixing down to an audio file. Once you're mixing down the arrangement, make sure to tick the option to export only the range between start and end marker in the export dialog.
Are you trying to pre-establish timing? This can easily be done but is often changed by the first sample. This requires re-setting the sample rate and timing. Give more detail if your question is still unanswered.