you may want to look around for a old primestar dish. They work well with a linear LNB and you can usally get them for free. Just put an ad in the local paper for dish removal service. Lots of people will pay you to take them down.
Well, you know I have 24x27 inches (thats between 60cm to 65cm) I still can get IA-5 some channels but most of channels are still missing.. I recommended ya to get at least 30 inches or so.