This may or may not work for length, but L.L. Bean has a white washable linen dress that comes in misses, and women's plus sizes in regular and petite lengths. It is kind of a shift dress, and yeah, linen after Labor Day may give some people the vapors, but for $49.50 it seems like a good deal. Also linen is cooler and in a crowded room full of women that may be a bonus. It allegedly comes to the knee (though that varies for each person) and has short cap sleeves. It can be washed and dried in a regular washer and dryer and LL Bean is great about returns and guarantees their products.
http://www.llbean.com/llb/shop/68023?feat=506213-GN2
Land's End has a white dress as well that is at least knee length but has no sleeves. It has sold out in quite a few sizes but is real cute. They are also great about returns, they can be done in a Sears store.
http://www.landsend.com/pp/EyeletDre...s&origin=index
Just don't forget appropriate undergarments like a slip, I'M SERIOUS
