Hi,
If you have the polygon geometry you can cast it to ICurve interface and get its Length, something like the following:
private double GetShapeLength(IGeometry geometry)
{
double geometryLength = 0;
if (geometry!=null&&geometry.GeometryType == esriGeometryType.esriGeometryPolygon)
{
ICurve curve = geometry as ICurve;
geometryLength= curve.Length;
}
return geometryLength;
}