Of course we all would like to have the biggest dish that could fit/be acceptable, however OP asked for the smallest dish that would do the job. I've found that make/break for people getting into C-band often revolves around the size of the dish. I wouldn't want to mislead someone into thinking a 1.8m would get every tough feed out there, but I also wouldn't want to discourage someone by insisting they must use a larger dish when a small one would suit the purpose.
58W is probably our most favorite bird and we keep our 1.8m on it almost all of the time. Its 7/8 FEC transponders are a walk in the park for us, and we have a fair amount of link margin - between 1-2 dB in the worst cases. The only time we have had reception problems in the past couple of years is when the dish got iced up and covered with snow. If only the lower third was covered, everything still worked fine.
I should point out we're near Denver and 58W is only at a 24 degree elevation, meaning our 1.8m is picking up some thermal noise from the earth. In PA the elevation is going to be more like 40 degrees, which should help reception. In terms of 58W's power contours, Denver probably receives a bit more power than PA, but the difference is probably on the order of 0.5 dB.