I don't believe there are any hidden flags or such in jamfHelper to manage the size of a drop down menu or even the size of the resulting window, which would help you get what you're after.
jamfHelper seems to use some internal mechanism to auto determine the appropriate window size depending on variables such as length of the description text, whether an icon is present and how large the icon is and some other stuff. I personally find it to be a bit too wild and wooly for my tastes. Nowadays I only use jamfHelper for basic messages or if I happen to need one of its special window modes like hud or fullscreen. For everything else I'm using cocoaDialog, which has more precise controls over window size and position on screen, if you choose to specify those.
