It's not about the length of the appeal, darling. It's about the quality of the writing.
Top professional letter writers worldwide see their best results with longer letters where they can tell a big and involving story. It's like watching a movie ... where you, the donor, are the star. Great professional direct mail embraces the reader, delights the reader, and empowers the reader to do something useful ... something that will help, heal or change the world. Long letters written by capable and talented hands take the time to raise the empathy of the reader to a boiling point.
Shorter is better if you're writing dull or dysfunctional letters.
The whole "how long should a direct mail appeal letter be" debate is irrelevant for most nonprofits because they write their stuff in-house or use semi-pro freelancers. Go for short and to the point. Best bet.