diff --git a/clang/include/clang/AST/Type.h b/clang/include/clang/AST/Type.h
--- a/clang/include/clang/AST/Type.h
+++ b/clang/include/clang/AST/Type.h
@@ -3253,6 +3253,10 @@
     return NumElements > VectorTypeBitfields::MaxNumElements;
   }
 
+  static unsigned getMaxNumElements() {
+    return VectorTypeBitfields::MaxNumElements;
+  }
+
   bool isSugared() const { return false; }
   QualType desugar() const { return QualType(this, 0); }
 
diff --git a/clang/include/clang/Basic/DiagnosticSemaKinds.td b/clang/include/clang/Basic/DiagnosticSemaKinds.td
--- a/clang/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/clang/include/clang/Basic/DiagnosticSemaKinds.td
@@ -2868,7 +2868,7 @@
 def err_attribute_invalid_size : Error<
   "vector size not an integral multiple of component size">;
 def err_attribute_zero_size : Error<"zero vector size">;
-def err_attribute_size_too_large : Error<"vector size too large">;
+def err_attribute_size_too_large : Error<"vector size too large. Size is %0 when the maximum allowed is %1">;
 def err_typecheck_vector_not_convertable_implict_truncation : Error<
    "cannot convert between %select{scalar|vector}0 type %1 and vector type"
    " %2 as implicit conversion would cause truncation">;
diff --git a/clang/lib/Sema/SemaType.cpp b/clang/lib/Sema/SemaType.cpp
--- a/clang/lib/Sema/SemaType.cpp
+++ b/clang/lib/Sema/SemaType.cpp
@@ -2453,7 +2453,10 @@
 
   if (VectorType::isVectorSizeTooLarge(VectorSize / TypeSize)) {
     Diag(AttrLoc, diag::err_attribute_size_too_large)
-        << SizeExpr->getSourceRange();
+        // Display sizes in error messages in bytes.
+        << SizeExpr->getSourceRange()
+        << static_cast<unsigned>(VecSize.getZExtValue())
+        << (VectorType::getMaxNumElements() * (TypeSize / 8));
     return QualType();
   }
 
@@ -2501,7 +2504,8 @@
 
     if (VectorType::isVectorSizeTooLarge(vectorSize)) {
       Diag(AttrLoc, diag::err_attribute_size_too_large)
-        << ArraySize->getSourceRange();
+          << ArraySize->getSourceRange() << vectorSize
+          << VectorType::getMaxNumElements();
       return QualType();
     }
 
diff --git a/clang/test/Sema/types.c b/clang/test/Sema/types.c
--- a/clang/test/Sema/types.c
+++ b/clang/test/Sema/types.c
@@ -70,8 +70,12 @@
 }
 
 // vector size too large
-int __attribute__ ((vector_size(8192))) x1; // expected-error {{vector size too large}}
-typedef int __attribute__ ((ext_vector_type(8192))) x2; // expected-error {{vector size too large}}
+int __attribute__((vector_size(8192)))
+x1; // expected-error {{vector size too large. Size is 8192 when the maximum
+    // allowed is 4092}}
+typedef int __attribute__((ext_vector_type(8192)))
+x2; // expected-error {{vector size too large. Size is 8192 when the maximum
+    // allowed is 1023}}
 
 // no support for vector enum type
 enum { e_2 } x3 __attribute__((vector_size(64))); // expected-error {{invalid vector element type}}
diff --git a/clang/test/SemaCXX/vector.cpp b/clang/test/SemaCXX/vector.cpp
--- a/clang/test/SemaCXX/vector.cpp
+++ b/clang/test/SemaCXX/vector.cpp
@@ -359,19 +359,22 @@
   // expected-error@#1 {{vector size not an integral multiple of component size}}
   // expected-note@+1 {{in instantiation of template class 'Templates::TemplateVectorType<int, 33>' requested here}}
   const TemplateVectorType<int, 33>::type BadSize;
-  // expected-error@#1 {{vector size too large}}
-  // expected-note@+1 {{in instantiation of template class 'Templates::TemplateVectorType<int, 8192>' requested here}}
+  // expected-error@#1 {{vector size too large. Size is 8192 when the maximum
+  // allowed is 4092}} expected-note@+1 {{in instantiation of template class
+  // 'Templates::TemplateVectorType<int, 8192>' requested here}}
   const TemplateVectorType<int, 8192>::type TooLarge;
   // expected-error@#1 {{zero vector size}}
   // expected-note@+1 {{in instantiation of template class 'Templates::TemplateVectorType<int, 0>' requested here}}
   const TemplateVectorType<int, 0>::type Zero;
 
-  // expected-error@#2 {{vector size too large}}
-  // expected-error@#3 {{vector size not an integral multiple of component size}}
-  // expected-note@+1 {{in instantiation of template class 'Templates::PR15730<8, int>' requested here}}
+  // expected-error@#2 {{vector size too large. Size is 8192 when the maximum
+  // allowed is 4092}} expected-error@#3 {{vector size not an integral multiple
+  // of component size}} expected-note@+1 {{in instantiation of template class
+  // 'Templates::PR15730<8, int>' requested here}}
   const PR15730<8, int>::type PR15730_1 = {};
-  // expected-error@#2 {{vector size too large}}
-  // expected-note@+1 {{in instantiation of template class 'Templates::PR15730<8, char>' requested here}}
+  // expected-error@#2 {{vector size too large. Size is 8192 when the maximum
+  // allowed is 1023}} expected-note@+1 {{in instantiation of template class
+  // 'Templates::PR15730<8, char>' requested here}}
   const PR15730<8, char>::type2 PR15730_2 = {};
 }