As I understand it the rigor required of a provisional patent [application] is less than that of a non provisional patent.
While this is true in a sense, there is a lack of nuance here. Provisional patent applications are not examined for formalities. Generally, this is interpreted as being able to omit claims. However, if you're really wild, you can even use a different page size.
However, a provisional still needs to be treated during the drafting stage as if it were a non-provisional. This is because the provisional needs to be able to support the non-provisional. Notably, while you do not need to file claims with the provisional, you should still have claims drafted for use in the non-provisional. This is because the description of the provisional needs to be able to support the claims of the non-provisional. If you insufficiently describe your invention in the provisional, it will be as if you never filed the provisional: it will be useless and you'll have lost a year of priority.
For this reason, it is not entirely accurate to say that the rigour of a provisional is less than that of a non-provisional patent application. Merely, the filing formalities and fees are lower.
What is the risk if the provisional is too narrow?
You will not have an earlier priority date for anything not described in the provisional. If you provide in your provisional that your invention must have features X and Y, then you would likely not have support in the non-provisional for your invention just having X alone. So be careful with what you describe as essential (as indeed you would with a non-provisional).
That said, if you avoid suggestions that features are essential, and take care to describe broader embodiments as potentially standalone, more detail is always better.
What is the risk if the provisional is too broad?
The risk is that the provisional will inadequately describe your invention. Claims in the non-provisional will therefore not be entitled to the benefit of the filing date of the provisional. Intervening prior art will therefore deprive your claims of novelty.
How do I get in the Goldilocks zone?
Dale M's suggestion of hiring a patent attorney, while flippant, is certainly the right answer.
But in general, more detail is better. As long as you describe your invention in its broadest embodiment without optional features, then describe each further embodiment while noting that the added features are beneficial but optional, you should be fine. Although this makes for a dull drafting experience, it makes for a strong provisional.