The Question Actually Quite Incomplete.

There is no such term Small or Big for an IT world. All what we need to know is features/Service which we want to add on in our project. That decides how much time it will take and how much developer it needs.
However Specifically if we talk about Flutter Framework, Its very Flexible and Open mode. Personally i like it very much as compare to other Native platforms. Well, if I would reply to your question by understanding your intention, then my reply would be Only One Developer, or Maximum Two.

Yes,

A full Stack developer , can complete your project alone, or might he/she need some one more to assist only if your project require so.(Depends on what you need to develop, A Mobile App, A Desktop Software or any website)However for a small project 80% chances are that, A single developer will complete your project.

You can catch such developer from HIND TECHNOVEL, A Leading IT Company.

Hopefully you got your answer, if any query remain then feel free to ask.
Thanks