I like your question and I'm too lazy to give you an authoritative answer (i.e., to find the actual acres for each resort). But I'd hazard a guess (based on plenty of experience with all three) that the three resorts are pretty close to the same in acreage. As for walking, I'm guessing the very longest walks would at OKW if you are going from the far SW end to the far NE end (near the Hospitality House). At SSR, even the furthest walk to the "hub" (from the Carousel area) is probably shorter than at OKW, although Carousel to the DTD end of the resort is a haul. At Coronado, the walk from the farthest Rancho to El Centro is certainly shorter than the very longest walks at SSR and OKW...but not by too much.
They're all big, but (for me) manageable.
