Hey,
First create a large floor (I use a giant plane). Then apply the Corona "Shadowcatcher Material" to the said floor. Inside the Shadowcatcher set the mode to "Enviornment", Alpha mode to "Always Solid". Then plug in a basic corona material that will define the background (Usually its just a white diffuse material). Open up the render settings and under "Scene environment" find "Direct visibility override". In there plug in that same basic corona material. Adjust that for the color of the background.
The plane is what is going to catch the shadows so technically you just need a "big enough" plane so that it covers all the areas where the objects will cast shadows. The basic material you'll apply will define the background which will match the floor so you won't see any seams of where the floor ends.
Hope that helps ;)