If you're having one transponder peak at a different spot than another, chances are that the issue is interference from an adjacent satellite. The bigger the dish, the sharper the focus, I think (assuming not a huge difference in focal ratios.) Of course, the sharper the focus, the better your tracking needs to be...
In this age of 8PSK and higher modulation, I wouldn't recommend less than a 90cm dish, although as others have said, some satellites benefit from a 1m or larger dish (primarily for feeds though, as broadcast channels tend to have more power.)
Finally, it's frustrating having a receiver that doesn't provide signal and quality readings in real units. An ideal receiver would display power, signal-to-noise ratio, and bit error rate. The last two are much more important than the first one.