Look at the Sea To Summit poncho - it seems to be the longest I've used, and the least in need of a cord around the waist. The Integral Designs Silponcho is also very good (probably my favorite poncho), but my not be long enough for you in back.

Having said that, I've pretty well given up on ponchos. I find I prefer a rain jacket and pants, mostly because they can double as wind garments. In the end, the only real solution to rain is to get out of it. I've never stayed completely dry in the rain in either a poncho or a rain suit, but they're a whole lot better than nothing.