The dinner portions of the filet and New York strip are larger than the lunch ones, but not dramatically so. I and my daughter had the filet at lunch there during our last trip and the portion -- a thick "baseball" style cut -- was more than adequate for our appetites.
In closing, if you want a serious, impressive piece of absolutely prime steak at WDW, forget Le Cellier, Yachtsman and all the other standards people repeatedly mention here and instead go over to the Dolphin and treat yourself at Shulas. It's quite pricey but worth doing at least once.