If you look at the examples at xbim docs, you will see how to get data for a space. To get finish floor height as defined in your link, you can use this code:
private static double? GetFinishFloorHeight(IIfcSpace space)
{
return space.IsDefinedBy
.SelectMany(r => r.RelatingPropertyDefinition.PropertySetDefinitions)
.OfType<IIfcElementQuantity>()
.Where(qs => qs.Name == "Qto_SpaceBaseQuantities")
.SelectMany(qset => qset.Quantities)
.OfType<IIfcQuantityLength>()
.Where(q => q.Name == "FinishFloorHeight")
.FirstOrDefault()?.LengthValue;
}