1. You can hold down the Shift key on your keyboard while resizing the arrow – this will however distort it. Or,
2. You can create the head of the arrow using the regular arrow shape, and then add a rectangle (the 2nd shape from the Geometric shapes group which is just above the arrow group) to act as the tail of the arrow. You will need to resize it to fit perfectly.
Hope that helps! Let us know if you have any other questions.