grammar - What are rules to reverse the position of subject and predicative when the former is too long

Two thoughts, depending on what you are actually thinking of

Thought 1: heavy CP shift?

Maybe you don't mean moving all the way to the front of the sentence, but just farther left?

The assumption is highly dubious [that a new airport will for sure make Tudor a flight hub].

Theoretically, this is known as a "heavy CP shift" (the CP is the chunk that I bracketed). This is basically the same thing as the more common heavy NP shift. The "heaviness" is the length of the constituent chunk of words; if the phrase gets too long, it is possible to "shift" it to the right. This makes it easier to parse the sentence. There is no set rule to how long the phrase must be, but the longer it gets, the stronger the "urge" to do the shift. The shifting is constrained by constituency, so we can't just shift some portion of a phrase or shift into the middle of another phrase, we can only shift proper chunks.

Thought 2: focus?

You can move "highly dubious" to the very front of the entire sentence, but this is not done because the other portion is too long; rather, it is done if you want to focus the "highly dubious" element of the sentence. Basically, this is done in order to say that a certain element is new information or something you want to give particular attention to.