Never -buy- a dish smaller than 36" (90cm).
There are single- and dual-output LNBFs, and you think you'll ever expand to 2..4 receivers in the future, consider the dual-output now.
(adding a multiswitch to a dual-output LNBF will let you feed 4..8 receivers easily - we have an FAQ for that when you are ready)
For first-timers, I'd install the dish about chest-height, so you can get to it easily for tune-up and future modifications.
That's often a pole in the back yard, or mounted to the side of the house.
You put it on the roof or any place that needs a ladder, and there's an 85% chance you'll be sorry.

The FTA hobby is one of learning, and growth, not static like installing DishNet or DirecTV.
These recommendations for SatelliteAV may seem a little self serving, as he's a site sponsor here.
Trust me when I say, sponsor or not, if someone messes with the members they will get chewed up!
One former sponsor certainly did.
Another former sponsor is still spoken of fondly and regularly, 'cause he did well by us.
I've called up Brian of SatelliteAV, and had a number of discussions with him over the years.
He and his company are very helpful, friendly, and seem to go out of the way to supply reliable equipment.
If his new receiver is available in a time frame that is convenient to you, I'd seriously consider it.
Past experience (mine and others) with his receivers has been very positive. - :up