(defun c:MleaderSizeChange ( / ss i textsize arrowsize)
(if (and (setq textsize (getdist "\nText size: "))
(setq arrowsize (getdist "\nArrow size: "))
(setq ss (ssget "_A" (list '(0 . "MULTILEADER") (cons 410 (getvar 'CTAB)))))
)
(repeat (setq i (sslength ss))
(setpropertyvalue (ssname ss (setq i (1- i))) "ArrowSize" arrowsize)
(setpropertyvalue (ssname ss i) "MText/TextHeight" textsize)))
(princ)
)
Edit: In case you have some Mleader with blocks instead of Mtext, the above will probably crash...
(vl-load-com)
(defun c:MleaderSizeChange ( / ss i textsize arrowsize)
(if (and (setq textsize (getdist "\nText size: "))
(setq arrowsize (getdist "\nArrow size: "))
(setq ss (ssget "_A" (list '(0 . "MULTILEADER") (cons 410 (getvar 'CTAB)))))
)
(repeat (setq i (sslength ss))
(setq obj (vlax-ename->vla-object (ssname ss (setq i (1- i)))))
(vl-catch-all-error-p (vl-catch-all-apply 'vla-put-ArrowheadSize (list obj arrowsize)))
(vl-catch-all-error-p (vl-catch-all-apply 'vla-put-TextHeight (list obj textsize)))))
(princ)
)