Lines and Arrows
Here is a pointer from the Scribus Archives about customizing the relationship between line thickness and arrow size.
If you use the automatic adjustments in Properties > Line tab, you will see that after you assign an arrow style, adjustments in line thickness cause an automatic change in size, illustrated here:
It's entirely possible that this size ratio doesn't suit you, so here is how to modify this behavior. First, adjust your line thickness and arrow so that the arrow size is correct. Now, create a line style, Edit > Line Styles (in 1.3.x, this will be in the new Styles Manager, Edit > Styles, then select Line Style).
Keeping it simple for this example, here are the settings for a 6 pt solid black line style, which we then save and assign to our line:
And here is the final result, with our new line style applied to all but the smallest arrow: