When I make lap quilts they're usually around 50-60x70 which is generous, and larger than most pattern books will show.
If you're doing it for a charity auction, etc. I wouldn't worry so much about size as colours and design.
That being said, it's really hard to know what would be appreciated by others as we often get surprises, don't we? :)
Another thing to consider is whether the group has had similar donations before and how well they have profited from them? Or if it's been a disappointment?
I'd hate you to put all your time, effort, and the cost into making your quilt and then discover that this group won't pay a reasonable $$ for your donation. Just something to think about!
Good for you for wanting to do this ... just be sure it's the "right" place and time, so you are not disappointed! :)