I thought implied target was already user resizeable, but taking a peak at the file the attribute isn't there, so...
Open up the eq2ui_mainhud_impliedtarget.xml file in your favorite text editor, and look for this object (it should be at the very top):
<Page DynamicData="/GameData.ImpliedTarget.Window" DynamicDataFilter="0001" eq2usescomwndcontrols="true" Location="640,134" MaximumSize="16000,135" MinimumSize="106,70" Name="ImpliedTarget" OnHoverIn="ConButton.visible=true
" OnHoverOut="ConButton.visible=false
" PackLocation="center,top" PackLocationProp="0000/0001,0134/0001" ScrollExtent="166,80" Size="166,80" UserMovable="true">
At the very end, right after
UserMoveable="true" add this:
UserResizeable="true" and that should let you resize it.
Mind you, you'll have to make sure that click-through and locked are both disabled first before you'll be able to resize it.