How to automatically resize the Dash when screen dimensions change?
Solution 1:
Here's a solution I think might help. There is a program called "MyUnity" and it is designed to let you customize Unity more than you would be able to with the default settings manager. There is a setting in that program to change the dash size. From what I have read online, there is a setting that maximizes the dash to whatever screen size you have. There is also a setting that does not ever maximize the dash.
You can install it with the following terminal command:
sudo apt-get install myunity
Once MyUnity is installed, open it and click the "dash" tab at the top of the window. The last option is for dash size. "Automatic" is obviously the default setting, and I think that is what is causing the current undesirable behavior for you. "Netbook" is the one that maximizes the dash to whatever screen size you are using, and "Desktop" is the setting that makes the dash a fixed size that is smaller than your screen size.
Please let me know if it solved your problem.
Solution 2:
nicktrip suggested MyUnity which will probably work; an alternative is Ubuntu Tweak. The dash size menu can be found under Tweaks > Desktop > Unity. No differences it seems, it also has three options: automatic, desktop and netbook.