In the Line Style menu on the Home tab, you can pick from a few basic arrowhead options. Click on Custom to set arrowhead size, displacement, and style.
Choose a Style
You can select the style of your arrow on both ends of your line. For example, if you wanted a very simple left to right arrow, you'd choose "None" for the left arrow and the basic arrowhead showing on the right side. Since you've selected custom, we assume you're looking for more technical arrowhead types used in engineering and software design diagrams to illustrate relationships. Select the appropriate style of arrow by clicking on it. The preview on the right hand side will show what your line will look like.
By default, arrows will touch the side of the shape they're connected to. You can create a displacement if you want to have a gap between the arrow and the shape. To do so, simply select the arrow with the space between the arrowhead and the shape.
Set the Size of Your Arrowheads
Unlike the style, the size of your arrowheads will be universal on both sides of your line. Choose the size that best fits your diagram. It will be applied to both the left and right, top and bottom arrows.