If the dim exists and you want to pick look at this, if you want to do dim and adjust straight away look at my other post using entlast.
(defun c:foo ()
(setq obj (vlax-ename->vla-object (car (entsel "\npick dimension "))))
(setq arrow (* 2.0 (vla-get-arrowheadsize obj)))
(vla-put-arrowheadsize obj arrow)
)
(c:foo)
Do repeatedly and arrows will grow.