I disagree with adding too many attributes to the sidebar. We should show only the frequently needed and leave everything else to the dialog. The particular case might be worth a discussion because of the overlapping arrows in case of small lines. On the other hand, don't you want to have a consistent layout with constant arrow sizes? In this case you should use a drawing style.

Principal point
It's not obvious that you can define the arrow head size. The sidebar isn't communicating it. So started looking in the 'arrow' head drop down for smaller size (but didn't find any; obviously in hindsight). The line dialog didn't come to mind. It's about accessibility of the feature. 
And well the sidebar is the first place to look; right. Having the most used setting available at hand. I don't size arrow head size as (A) some advanced setting (b) barely to be used

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Other matter: Drawing toolbar different variations of "Lines and Arrows". Those are pre-defined line "styles"; sort of template. However not 'a style' in the sense a style in the style deck. Which is - while hammering on using styles - terrible inconsistent :-). The offering pre-defined lines and arrows, without being a actual style. 

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Now lets assume: The various lines and arrows shown in the "Lines and Arrows" would be actual Styles. 
(A) The side-bar side-deck would - out of the be box - already raddled by styles. Becoming worse when adding a number of customizations for the various lines to be used (red line, blue arrow, different type of arrow)
B) You also need to add text labels to all those styles, and find the style by label; not preview :-(
C) If you designing a new arrow (as DF), which - without you realizing - matches an existing style: a warning occur that the line/arrow layout already being defined as a style; with option to convert

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Styles (in general, not specific for Draw)
I'm bit allergic for pointing to styles in each and every context. Styles are - similar to templates - 'normally' used in professional setup.

Corporate
* Using house styles for internal or external audience. 
* Same sort of documents being created by dozen of employees)
* Same type of document being generated thousand of times

Small medium businesses (or non-profit organizations, writers)
Say a user manual for peach of software: manual written by (say) a single person; audience: end-user. Manual might be updated for years by the same person (or someone else)

Home user
* documents generated for single use (no repeat usage)
* Often for personal use, not to be published. Or only to be read by small group of people.
* Documents are normally small (1-4 pages)

Consequence
* Setting up a whole style being pointless (not to be re-used). No interest in investing lots of time. 
* Quick and dirty documents. It needs to get the job done, no awards. Priority is to be functional.

The essence: I except tool to be workable out of the box. Not going to many (distracting) hoops and loops.

@Telesto: Is the main problem the fact that making the arrow short results in overlapping arrow-heads, or the ability to specify arrowhead sizes in the sidebar?

If it's the former, perhaps the proposal might be making the arrowhead default sizes be arrow-length-dependent?

@Heiko: For a 1D object, would you not say that the Line controls are rather fundamental? I think the arrowhead sizes should fit in there.

At least, there could be some mechaniosm for opening the line's properties on the Line pane from the drop-down list  for head types. But I think I would probably be in favor of having the arrowhead sizes on the sidebar as well.

(In reply to Eyal Rozenberg from comment #3)
> @Telesto: Is the main problem the fact that making the arrow short results
> in overlapping arrow-heads, or the ability to specify arrowhead sizes in the
> sidebar?

A) It started with the need of drawing a short line (with the mouse) with a arrow at both ends, and concluding those arrow-heads overlapping. So this is the 'main' issue

B) As a result I started to look how to solve the problem. So looking through the list of arrow heads in the for a smaller one, I the assumption it might list a pre-defined sizes of the same arrow. A bit similar to how you can choose between line thickness; it also gives a list of options/previews instead a just an spinbox for input.

I didn't even consider that this could be controlled by value... Makes sense in hindsight. Hence request for adding it to sidebar. 

C) Other curious fact: if you adjust the line thickness the arrows increase/decrease too. Nothing saying this is undesired in general. But there nothing in the UI suggesting arrow heads size being adjusted in sync with line thickness. There is no way to adjust the line thickness without the arrow-heads