Since we've recently had at least a couple of users using the wrong dish size, let me try to walk you through a simple site that allows you to find the right dish size for you.
DISCLAIMER: Outernet is not affiliated with or sponsored by SatBeams, it does not endorse it, and information you find there are for informational purposes only. If you buy a dish of any size based on info you find there, and it doesn't work, we're not responsible, yada yada yada.
Before you start, you need to know your location on the world map (try Google Maps search) and the orbital position of the satellite you want to tune into which you can find on the Coverage page in our wiki. We'll give you an example for Chicago, and Galaxy 19. Galaxy 19's orbital position is 97.0W.
The site is called SatBeams and contains useful tools for working with satellites. The tool we are interested in is the footprint map. It is available through the Footprint tab on their main page.
There are two ways to locate your satellite on the map. One is using the search feature. We won't go into that here, but it's not too difficult to use if you spend some time poking around. Another is to use the satellite belt above the map, which is what we'll use here. Find the satellite icon that is at the orbital position and click it.
In some cases, multiple satellites or same satellite emitting multiple bands can be found at your orbital position. In case of Galaxy 19, we have C and Ku band at that position. You need to know exactly which one to choose. In case of Galaxy 19, we need Ku band (that info is also available on the coverage page, btw).
Now navigate around the map to find your location. When you've found it (it doesn't need to be super-precise), click on it. A info box pops up with details about reception at that location.
If you are a sat guy, you'd know what EIRP means. If you're not, then you can use the 'Recommended dish size' information shown below it. We also (unscientifically) recommend adding around 10cm (~4") to the dish size on SatBeams to compensate for less-than-ideal environment (clouds, minor obstacles, etc).
Now, please not that dish size is not the only factor that affects your reception. You also need a clear view of the sky at the location where your satellite would be. You also need to point your dish correctly. You don't want cables that are too long (those tend to degrade the signal). And so on.
I hope this has been helpful to you.