You do not provide enough information to make a determination. These days, you also need to consider the thickness and the rigidity.
If less than 1/4" thick, a 6x9 would still be classified as a Letter.
If over 1/4", then it depends on if the package is uniform in thickness & flexible. If so, then it is a Flat/Large Envelope.
If over 1/4" and lumpy or rigid, then it is a Parcel.
In any case, if it is greater than 3/4" at any point, it is a Parcel.