[DF] fix accesing RNTuple R_rdf_sizeof_ columns as size_t

Author: jblomer

tree/dataframe/src/RNTupleDS.cxx

@@ -21,6 +21,7 @@
 #include <ROOT/RNTupleDS.hxx>
 #include <ROOT/RNTupleTypes.hxx>
 #include <ROOT/RPageStorage.hxx>
+#include <ROOT/StringUtils.hxx>
 #include <string_view>
 
 #include <TError.h>
@@ -517,6 +518,12 @@ ROOT::RFieldBase *ROOT::RDF::RNTupleDS::GetFieldWithTypeChecks(std::string_view
       return fProtoFields[index].get();
    }
 
+   if (tid == typeid(std::size_t) && ROOT::StartsWith(fProtoFields[index]->GetFieldName(), "R_rdf_sizeof_")) {
+      // The R_rdf_sizeof columns should always be accessible as std::sizeof, even of there normalized integer type
+      // differs (e.g., "unsigned long" vs. "unsigned long long")
+      return fProtoFields[index].get();
+   }
+
    // The user explicitly requested a type
    const auto requestedType = ROOT::Internal::GetRenormalizedTypeName(ROOT::Internal::RDF::TypeID2TypeName(tid));