How tall do you need these 'trees' to be?

I wouldn't actually recommend them, but Leylandii grow very quickly, and very tall, but are difficult to control. When I moved in to my house 18 years ago, I cut several down in my back garden - thank God! My neighbour has a short hedge of several of them. They are about 6 or 7 metres high, and he has to have a gardener come in about once a month to cut it back to prevent it taking over the neighbourhood!

Just remember that trees that grow quickly don't stop when you want them to and can result in a lot of work trimming them. Many of the conifers are nice looking but can suddenly develop brown patches that never go green again and honestly I'd avoid them. How high do you want your hedge and how much width can you spare?

Even Leylandii won't like wet soil, I can't think of a conifer that does, frankly. Even if it did, unless you want large bills every two years from having them kept under control, I wouldn't recommend it. Which is why I asked how tall the plants need to be - there are shrubs which will tolerate wetness.

I think that there are Ilexes that will tolerate wet soil.