The Width is set by the default ScreenTip Style and the default template has a trigger to change the Width in a certain scenario. The widths are properly set to meet the Microsoft ribbon requirements for ScreenTip sizes.
You may be able to create an app-wide Style for ScreenTip that sets the Width in a Setter. However if no ImageSource is set on the ScreenTip, that template trigger may still set it to 210, overriding your setter.
If the app-wide Style with single Width Setter doesn't work, you could clone our default Style and use that (including a clone of the default template) as your app-wide Style.