novice:
You could try saving the mesh as an Object (use the Top View in the 3D Window), and then adjusting the height of the object, which should rescale the object's height proportionally. The downside is that it will no longer be an editable Mesh, but a mostly fixed object.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David