If I read your question correctly, there is no way to change region size. However, it does not have to be 11 tall. If you position the center chest (where you create the region itself) on the floor, then it will also cover the area below the building. Or, say you put it on the upper part of the building, it would then region the air above the building, as long as you have the required stuff within the range. Whatever position you do, the place where you stand while typing the command will be the center. From that point, the region will be 5 below you and 5 above you. I hoped I helped in some way!
