@ceilr – Peter is right, there is no direct control to adjust the size of arrow heads in InDesign.
However, there is a not well understood algorithm, that controls the size of an arrow head in relation to the stroke weight of a graphic line. The arrow head does not grow direct proportionally when adjusting the stroke weight.
See the following screen shot, a graphic line with an arrow head:
Stroke weight of 0.1pt against 10pt:
As you can see there is a significant difference here. The size of the arrow head does not grow in direct proportion to the stroke weight.
Around 42:1 (left sample) vs. around 2.3:1 (right sample) if you compare the base of the triangle of the arrow head to the thickness of the line.
So, if you decide that the proportion of the right arrow head in comparison to line thickness is alright for you, but the stroke weight should be 1 pt (instead of the 10 pt currently) while maintaining the proportion of the arrow head to line thickness, you could scale the graphic line to 10%.
However to do this successfully you should set your General Preferences "When Scaling" to: "Adjust Scaling Percentage".
See the following screen shot where the 10 pt line is scaled to 10% after setting the General Preferences:
Set your preferences back after scaling…
That's very clever, Uwe.
Actually, very very well done!
@Peter and Obi-wan – thank you both…
However this solution is a bit fragile and has some implications that makes it hard to see or to control the effective stroke weight of the graphic line directly in the GUI.
You always have to do some calculations. There is no GUI specific "Effective Stroke Weight" display for scaled objects.
And: Never do "Redefine Scaling as 100%" in the Transform Panel.
... but we can take a little time to create a Library of validated good strokes to be reused. So, for me, it's good ... and cool!
Hmmm, I will check this out!
Perhaps my other option is to create just an arrowhead in Illustrator and import it to InDesign. At least that way I will have something to scale when I need a smaller arrowhead on a fatter line.
OR, I can draw an arrow head by hand in ID, color the fill, and set the stroke to zero. All the lines I have that are in need of arrows are horizontal, so this way I can make sure that the handle at an arrow's point aligns with both handles on the horizontal line.
Thank you, everyone, for your varied and helpful answers.