All satellites have what is called a "footprint" and the further you get away from the center of the footprint or beam, the weaker the signals are going to be, and the large of dish you will need in order to receive a clear picture. Once you get past the line of sight view to a satellite the signal is completely lost and no size of dish will be large enough to capture enough signal in order to get a usable picture and/or sound from that satellite. This works much like the beam of light coming from a flashlight in a dark area.
For most areas of North America, a 7.5' to 8.5' dish is needed to receive good analog C-Band reception, while it is recommended that a 10' dish be used to receive digital satellite transmissions.
Besides performing a site survey at your house to see if you have a line of sight view to the desired satellites and no trees or other obstructions are blocking the signal, you can visit the satellite operators web sites and look at the footprint of each of their satellites. Here is a list of the North American satellite operators and the satellites that they own. The links will take you directly to a list of their satellite fleet, where you can view the information.
C-Band coverage for Intelsat Americas 5, Courtesy Intelsat