You will need at least a 30 inch (75 centimeter) dish to pick up most KU band FTA satellite signals. If you want to use C band you will probably need at least a 6 foot dish.
And like in most things, the minimum size can be occasionally frustrating. I'd recommend no smaller than a 90cm dish for Ku ( 1.0M if you expect to motorize and get DVB-S2 without a lot of trouble).
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