Two cents worth from a "hefty" user....

I have been launching a Sailaire (149"; 9.5 lbs.) and a Windsong (and a Lovesong) (132"; 5 lbs.) for many years using a 50/150 combo. The rubber is the largest size that Hobby Lobby sells and the string is #18 carpenters twine (orange for good visibility). Both of these planes have rounded fuselages behind the wing so it is easy to wrap the full hand around it and give it a heave. I haven't measured the tension but it is probably in the 15 to 20 lb range. The typical launch angle is about 35-40 degrees (I could probably increase that but I'm just not as strong as I used to be) and I provide as much arm as I can to get that initial flying speed. I can expect about 1 in 10 launches to have some sort of excitement due to a wind shift or a heavy thumb. I used to use a 100/300 setup but found that the extra launch height wasn't worth the extra trouble associated with that much length. You need twice the space and if the wind isn't in your face, the parachute can easily end up in the next area code. I also found it much more challenging to hunt for a thermal at the 150-200 foot level.