updated 9/10/09

Dialog Box Design Rules 

Sizing & Spacing

Dialog boxes should adjust to fit the content they're presenting to the user.
Sizing and spacing should be as follows.

Dialog Fonts, Icons & Rules

Dialog headline: Heading 1
Exception: If the headline is too long, and cannot be shortened, the headline may be set to Heading 2.

Dialog text: Heading 6
Dialog text links: Heading 6, Primary Link formatting

Exceptions 

If the dialog box will lead into another one directly and the size is pre-determined, then the sizing rules above will not apply.

Example: Insert Image Widget dialog > File Manager

What not to do
Do not make a dialog or warning box larger than it needs to be.
Example below.

New Dialog box design proposal (draft)