To all the parameters that you can change on a group node, you have some others on top, that helps you decide where exactly you want to put your bounding box and what size it should be. And of course you can choose whether it is the object that comes into the object, or you define another one by checking "Use Reference Object" and just pull your object in. In the end, you have a multiplier on the right side to overshoot your bounding box in position and size to get exactly what you want.