See if can extend the range of the scale sliders to work with current volume spacing (check for clamping).
Another option is to provide an entry box to reset the maximum size manually, but that would be another display parameter to track.

Would it work well enough to only expand the maximum scale settings on entry of the Markups module?

My first implementation was checking each time the widget was updated from mrml, but that's happening often and the logic is a bit tricky to restrict the scale max update - the correct way to do it would be to add an observation on the current volume node and check for spacing changes, but that involves a lot of book keeping.

So I'll work from the assumption of this workflow: a volume is loaded with spacing N, a markup is added and is too small, the user goes to the markups module and can now set the glyph scale to a useful value.

I also added checks that will increase the maximum if a fiducial list is loaded/modified to use a scale larger than the current scale slider max.

I'm using a scale factor of 5.0 x the maximum slice spacing - let me know if that works for you.

Here's the current state:
https://github.com/naucoin/Slicer/commits/3547-match-markups-scale-to-volume-spacing

That sound like it should work. I think 10x spacing for the max should work better.

Updated the git topic branch, will integrate to the trunk as soon as possible.

Pull request:
https://github.com/Slicer/Slicer/pull/94

Commited as svn 22919:
http://viewvc.slicer.org/viewvc.cgi/Slicer4?view=revision&revision=22919

Here is a sample volume:
http://placid.nlm.nih.gov/folder/286

The image above is 1005x1004x296 with spacing of 3x3x3.

I think I naively simplified this problem to just spacing. It may be better categorized by a "wide field of view". For the above image I can scale the 3D fiducial to 30X and it'll look OK, but then the 2D fiducial is too large.

Retarget stale issues to backlog.

Markups fiducials size can now be relative to screen size (this is the default).