It appears that support had been dropped for configuring the dash size in the latest Unity release, and now the Desktop option has no effect (i.e. Netbook size is always used).
Just to illustrate, this is what I get regardless of the screen size or the configuration:
I have tried to set it via Ubuntu Tweak, like so (see Dash size):
And the Unity Tweak Tool is already ignoring that option altogether:
What's more, the form-factor entry is missing from dconf-editor:
I really like a. having the dash spreading on my entire desktop, and b. being able to configure it (!). Is it really gone?
Best Answer
It seems that 13.04 has moved to Unity 3D out-of-the-box, hence no 2D features (like Dash size) are available.
The only way, than, to change the dash layout would be to maximize / restore it via the window handles on top: