I have made a number of quilts over the years for fundraisers in which the quilt was either raffled or auctioned. The raffles did actually bring in more money, but that was mainly because tickets were sold over a period of time. But if the raffle is not an option, then you just accept it.
As far as size, I personally don't offer anything less than queen size. A runner or lap quilt is so limited in size that many people wouldn't bid on it, no matter how beautiful it is. But a queen size quilt would appeal to a large number of people. I find that the general public still mainly associates quilts with bedcovers, something to keep you warm. Many don't think, as we quilters do, that there are so many reasons to have a quilt (bed runner, table topper, wall hanging, etc). So I feel that a large quilt would appeal to the largest number of people.
For the actual look of the quilt, be sure to use colorful fabrics. Those seem to spark the most interest.