var selectedElementId = _selection.GetElementIds().FirstOrDefault();
var familyInstance = _document.GetElement(selectedElementId) as FamilyInstance;
View3D view3D = new FilteredElementCollector(_document)
.OfClass(typeof(View3D))
.Cast<View3D>()
.FirstOrDefault(v => !v.IsTemplate);
Options geomOpts = new Options { View = view3D };
var symbolGeometry = familyInstance
.get_Geometry(geomOpts)
.OfType<GeometryInstance>()
?.FirstOrDefault()
?.GetSymbolGeometry();
XYZ symbolMax = symbolGeometry.GetBoundingBox().Max;
XYZ symbolMin = symbolGeometry.GetBoundingBox().Min;