Any way to force lightmap dimensions?

I know I can play with the 'resolution' setting, but what I'm hoping for is a way to say '256x128' or whatever size I deem good. I tried resizing these exr files, but the plugins I have are not good enough for that.

You can set these from script: http://unity3d.com/support/documentation/ScriptReference/LightmapEditorSettings-maxAtlasWidth.html http://unity3d.com/support/documentation/ScriptReference/LightmapEditorSettings-maxAtlasHeight.html

As far as I know there is no finer control.