Yes, it is possible. Highlight the line end that you want changed from a large arrow to a small arrow and the find the "Line End" tool (a drop down selection) from the Formatting Toolbar. Select the arrow size you want.

FWIW, I am using Visio 2003...an older version but, and yours may be a little different (not sure). In any event, you are looking to change the "Line Ends".