The Sat antenna you use it totally dependent on the kind of sat card you have. Almost all DVB cards can be used with a generic sat dish of as little as 12" diameter, if all you want to aim at is a single sat at a time. An 18" "elliptical" disk can be used to pick up multiple sats's without re-aiming. Several DVB cards also support driving a pointing motor to allow the dish to be mechanically steered to different sats.
A related issue is the LNB, the electronic device that takes the concentrated microwave signal from the dish and sends it down the wire as an electrical signal. Multiple LNB's can be used if you use a DisEqC box to switch between them; this kind of switch is directly supported by many DVB cards without any extra wiring.