A lot of the concepts around working the OSCAR 100 is a bit different than your typical amateur radio station. One of the differences is the introduction to using a satellite dish. If you have never played with microwave frequencies then chances are you have never used a dish in your station.
My general experience with the dish size is that in this case, “Bigger is Better”. You do however have a range of options. For the narrow band transponder you can easily go as low as a 60cm dish for both receiving and transmitting (a lot of guys have portable stations using a 60cm dish). For starting out I would suggest something in the range of an 80cm dish. If you have a dish lying around then start with that before you go and buy one. You also don’t need to be limited to 1 dish. Some stations use a dish for receiving and a dish for transmitting. One thing to keep in mind that the smaller your dish is the more power you will need when transmitting.