Will grow much bigger than 6'. During a coming winter dig it up and put it in another spot, where it can develop and be enjoyed as an unconfined tree - do not start whacking it back, this will spoil it. In my area Washington hawthorn often produces an interesting branch pattern and loads of conspicuous fruits when happily situated.

Mine suckers like crazy and could easily be kept as a bush with semi annual pruning. If yours shows it wants to be a tree, do as bboy recommends and move it when it goes dormant this fall.

...or chop it off right above ground level and it will become something more akin to a shrub. Trees don't always have to have their "natural form". Sometimes we modify them (think bonsai or topiary) to meet what we want them to be. That can be an uphill climb, in some cases, but if it's what you want, it's what you want. You could always grow another cutting if you decide you want a tree somewhere else.