Dissolve yeast and sugar in warm water in a large bowl; let stand 5 minutes. Lightly spoon flour into dry measuring cups, and level with a knife. Add 5 3/4 cups flour and salt, and beat with a mixer at medium speed until smooth. Turn dough out onto a floured surface. Knead until smooth and elastic (about 10 minutes); add enough of remaining flour, 1 tablespoon at a time, to prevent dough from sticking to hands (dough will feel tacky).
Place the dough in a large bowl coated with cooking spray, turning to coat top. Cover and let rise in a warm place (85°), free from drafts, for 45 minutes or until doubled in size. (Press two fingers into the dough. If an indentation remains, the dough has risen enough.) Punch dough down; cover and let rest for 5 minutes. Divide in half. Working with one portion at a time (cover remaining dough to keep from drying), shape each portion into a 12-inch loaf on a floured surface. Place the loaves on a large baking sheet coated with cooking spray and sprinkled with cornmeal. Cover and let rise for 15 minutes.
Uncover dough. Brush the loaves with cold water; make 5 diagonal cuts 1/4 inch deep across top of each loaf using a sharp knife. Place baking sheet in cold oven, and set to 400°. Bake 40 minutes or until loaves are browned on bottom and sound hollow when tapped. Remove from pan, and cool on wire racks.