To work out the size of garden room you require, you have two options: work out the size of garden room the garden will allow without it looking too imposing or decide what you need the garden room to house (i.e., a particular number of people to comfortably entertain) and then design the garden around the garden room.
This garden room has been designed with wood to the right of the structure, which provides a screen and backdrop for the tree ferns. The colour of the wood is a little darker than would normally be used. This is so that it provides a nice contrast and accentuates the green of the lawn and vegetation.
The darker colour of the wood also allows for a nice bright white gravel. It is a good example of how well a garden can work with a garden room, especially when the two are designed in tandem.