Hi Tricia,
You can always set arrow size for already created leader in Properties Palette, just select leader object, verify that "All" properties option is active on the top of the properties Palette and look for arrow size property.
If you need to set arrow size for newly created leaders -it all depends on what command you are using:
- for leaders you want to create with MLEADER command you need to modify current MLEADER style (menu Format -> Mleader Style -> Format tab
- for leaders you want to create with LEADER command you meet to modify current Dimension Style: add subdimension style for Leaders (if you don't have it), then modify arrow size for this style.
I'd suggest you to use MLEADER command, the leaders created with it are more flexible and have more editing options:
http://help.autodesk.com/view/ACDMAC/2017/ENU/?guid=GUID-764DA12B-1280-4D1A-8673-F9F8A136CB83#GUID-7...