If I understand what you are asking, you simply click and hold the bottom right-hand corner of your Beatbox object in the arranger and pull it to the right as far as you need it to be.
You can use this same procedure at either end of any object to shorten or lengthen it. When the length of the object exceeds the length of the loop, it simply repeats itself.
When altering an object to an interval not marked by the grid, it is always a good idea to zoom in as much as necessary.