I have become real fond of 200-250 lbs test Dacron fishing line. It is about 1/3 the size of 550 cord or 2-3 times the size of one inner cord of 550 but very very strong.
One thing you need to know is that, this is what's called a FLAT braid, and not a round braid. You can feel the difference if you roll it between your fingers, but that is it. It really doesn't make any difference at this size of cord.
I buy it in bulk off of ebay pretty cheap.
Now if you want something even stronger, like 500 lbs test, and is just under 1/16'' thick by .010'' or so, get SPECTRA line from Windstar Kites.
WSK PREMIUM SPECTRA Approximate Diameters and Weights:
TEST (Pounds) Diameter Weight per 1000'
40# .010" 1.2 oz. / 1000'
80# .015" 2.3 oz. / 1000'
130# .020" 2.86 oz. / 1000'
200# .023" 4.17 oz. / 1000'
300# .035" 8.75 oz. / 1000'
500# .051" 12.75 oz. / 1000'
1000# .112" 38.4 oz. / 1000'
http://windstarkites.com/FlyLine-SportandStuntKiteLine.htm