Magnolia Grandiflora trunk diameter?

[gringlejingle]

We've just had the back of our garden re-landscaped, with two 3m Magnolia Grandifloras planted close to our 2m high back wall for screening (we're overlooked by flats).

The trunk diameter is currently only about 10cm and the trunks are approximately 50cm from the wall. I'm now worrying that they're too close but can't find any relevant guidance online. We would eventually like the trees to grow to about 12m tall over many years, but I'm wondering how big the trunk diameter will be by then?

Does anyone have a 12m+ magnolia and, if so, how wide is your trunk?

It's not so much the trunk width that might be a problem being that distance from a fence, but the spread of the branches - they can get very wide and usually have multiple trunks. They take up to 50 years to reach that ultimate height you mention.