"Is this overcoat too short?"

Translation: Czy ten płaszcz jest zbyt krótki?

[DrStotter]

Would it be correct to use "za" instead of "zbyt" in this context?

[Jellei]

Yes. They are not always interchangeable and it's not that easy to see the borders of when they are and when they are not; but it would work here without any problems and it is accepted.

[kissamakea]

I wrote "Czy jest ten płaszcz za krótki". How bad is it to change the word order?

[Jellei]

Yeah, this sounds rather strange. As it is the overcoat that is too short, "jest" should stay between them.

[j4zzyy]

How come when I forgot the jest it was marked wrong?

[alik1989]

My guess is as good as yours, but I'd say it's because you forgot the verb.