Would it be possible to use trusses to span the 22' distance and hold up the loft floor? Seems to me that you might be able to design 2 trusses (one at the back wall and one at the front of the loft) over the loft area, and look better than a lot of 8x12 (heavy) floor joists over the back half of your great room. If the trusses carried the load, you could reduce the size of your loft floor joists, as they'd only be 12' long, running from truss to truss.