I have bought a Magnolia stellata that has a height of 27 inches (70 cm) above ground and a root system 8 inches deep (20 cm). It looked like it was newly potted, so I went home and checked the roots. From what it looks like, the roots have been cut to fit the small pot. Based on this information, can you tell what is the maximum height this plant will reach?
In the ground, in 10-20 years, it should have a height between 1.5 to 2.5 metres, with a spread of 2.5-4 metres. The variation in possible size is down to weather conditions, soil suitability, and availability of water.
That assumes, if the roots have been pruned, the plant grows away quite happily in its new situation. If too much root material has been removed, it might not make it, or will limp along for a considerable period before recovering. Provided it recovers, ultimate height and spread will be the same, in other words, the root pruning might make a difference to speed of growth initially, but will have no impact long term.