My guild has an annual auction in conjunction with our outdoor quilt show. The larger quilts definitely get more bids and go for a higher price, but I don't know whether they're selling for more per square inch. In other words, would four 50x50 quilts sell for more than one 100x100?
If your fundraiser is for a charity that deals with animals, then quilts with animals will probably do well. If the audience is likely to be interested in cars, then make a quilt with cars. Try to make a quilt that matches the interests of your bidders, if that's possible.