I am using Ubuntu Natty with the latest updates and the Unity Dash is in full screen. My laptop has a 17" screen with a resolution of 1366 x 768. If this is not the normal behavior I will report it as a bug on Launchpad but I'm not sure of the specifics if anyone could tell me what the specifics are regarding the Dash size.
If you want to disable Unity Dash full-screen mode, just type in a terminal:
gsettings set com.canonical.Unity form-factor 'Desktop'
For more details, see this post.
Someone has already posted a bug report about this at launchpad: