Pruning rose trees are similar than pruning climber roses. You want to prune to shape and remove dead canes. Do not cut the main vertical stock of the tree (I will call it the trunk). You can use the length from the graft to the vertical top as the graft point center radius length for pruning arount the tree. If you think you cut too much off no worries, it will grow back. You decide the shape, circle, umbrella, as long as it is symmitrical. An unballanced tree will cause uneven distribution of weight that could damage the trunk.