I had a 12 by whatever one at my old house. I had a fan trained peach tree at the gable end, and a grape vine going the length of the eaves, with the fruiting shoots trained up over the inside of the roof. Great shading in the summer, and would help to 'hold it down' in the wind. We had loads of peaches, I was very sorry to have to leave it, especially as the new owners got rid of the greenhouse and cut the peach tree down. Definitely go for the biggest you can afford/fit in. I've had to downsize to 8 by 6, not big enough to house a tree and a vine
