Another tech ran into something similar.
Here are those notes:
A couple additional suggestions, that would be good to try.
First, make sure the layout is assigned wireframe and not another display mode.
In Rhino, under the Rhino menu -> Preference -> Display models, edit Wireframe .
Pick restore to defaults in the upper right corner:
I just feel the display mode is in play here. The arrow changes size if the displaymode is Rendered or Pen which is very odd.
But check your print settings, too. I think it was set to 2mm which is very tiny…
I set the arrow 10mm. You can pick cancel and it will stick.
Other than this the arrowhead can be difficult to get to get the correct size and keep the correct size since it is dynamically scaled. You can make your own arrow head and insert it as a block. Not as convenient as the arrow head, but it will not dynamically scale.
Another options is to try the Rhino 7 WIP and see if the Arrowhead is scaled appropriately there. It would just give us additional information and may lead us to a solution.