Let us first give you some US statistics:
The average size green in the United States is 72 square metres.
In the northeast the average size is 98 square metres,
the south east 78 square metres,
the mid west 52 square metres and
the west coast 75 square metres.

The top teachers in the world say the ideal practice facility should be at least 10.5 metres in length as the average golfer does not often practice ‘lag putts” and the distance of 10 metres across your green will give you this ability, This information is provided only as a guideline for your facility, as your needs might differ greatly from these. Go into your backyard decide where you want to place your green, then take a hose, a rope or landscape flags and ‘map’ out your green. Once you have done this measure the length and width to get an idea of how big your facility will be.