I think a good general rule that always seems to work is having 1/3 patio, 1/3 lawn, 1/3 planting area. Obviously this is just a guide line, every property is different and have their challenges and needs like more lawn for kids or less because space constraints or less maintenance or water requirements.
Having all three principles in the garden is important regardless of the ratio. But if I was going to pick one of the three to increase or decrease from the ratio it would be the lawn, its the conduit that ties the garden together and makes it flow.
The patio needs enough space for at least two separate seating areas to give the garden seating options and the feeling of more room, this can be done in small courtyard areas as well. And the planting areas need enough depth to have some sort of design meaning, allowing you to enjoy the potential of a beautiful garden.