are you shore that you want to reshape your acer griseum, they have a shape of their own which is a part of their attraction.inserdently it will grow six to twelve inches each year for the next seven years, I would tend to leave it ,but if you have too, then during the dormant time december january and cut as near to it's origen as you can ,(leave no stumps) and paint.

Personally I agree with Cliffo and would not prune.

Thanks Cliffo and MoonGrower. This certainly makes me reconsider - it was just that I wanted to make it look more like a tree and less like a giant shrub with branches coming out so low down on the trunk. BUT it is so beautiful at the moment - brilliant red leaves and glorious orange peeling bark. It's obviously very happy where it is, putting on at least 1-2 feet since last autumn! So much for being assured by the nurseries that this is a small tree - but I suppose it is really, for a tree. But it is 15 feet high now in just 8 years!

I think it would really spoil the look of it to prune so if you can manage not to it would be best.