diff --git a/ironic_inspector/plugins/standard.py b/ironic_inspector/plugins/standard.py
index 8c9befe24..83601d799 100644
--- a/ironic_inspector/plugins/standard.py
+++ b/ironic_inspector/plugins/standard.py
@@ -53,6 +53,15 @@ class RootDiskSelectionHook(base.ProcessingHook):
                   'as an inspection ramdisk'),
                 node_info=node_info, data=introspection_data)
 
+        if 'size' in hints:
+            # Special case to match IPA behaviour
+            try:
+                hints['size'] = int(hints['size'])
+            except (TypeError, ValueError):
+                raise utils.Error(_('Invalid root device size hint, expected '
+                                    'an integer, got %s') % hints['size'],
+                                  node_info=node_info, data=introspection_data)
+
         disks = inventory.get('disks', [])
         if not disks:
             raise utils.Error(_('No disks found'),
diff --git a/ironic_inspector/test/unit/test_plugins_standard.py b/ironic_inspector/test/unit/test_plugins_standard.py
index 2892a88f5..b5f76b363 100644
--- a/ironic_inspector/test/unit/test_plugins_standard.py
+++ b/ironic_inspector/test/unit/test_plugins_standard.py
@@ -388,6 +388,19 @@ class TestRootDiskSelection(test_base.NodeTest):
         self.assertNotIn('local_gb', self.data)
         self.assertNotIn('root_disk', self.data)
 
+    def test_size_string(self):
+        self.node.properties['root_device'] = {'size': '10'}
+        self.hook.before_update(self.data, self.node_info)
+        self.assertEqual(self.matched, self.data['root_disk'])
+
+    def test_size_invalid(self):
+        for bad_size in ('foo', None, {}):
+            self.node.properties['root_device'] = {'size': bad_size}
+            self.assertRaisesRegexp(utils.Error,
+                                    'Invalid root device size hint',
+                                    self.hook.before_update,
+                                    self.data, self.node_info)
+
 
 class TestRamdiskError(test_base.BaseTest):
     def setUp(self):
diff --git a/releasenotes/notes/size-hint-ea2a264468e1fcb7.yaml b/releasenotes/notes/size-hint-ea2a264468e1fcb7.yaml
new file mode 100644
index 000000000..e75afa397
--- /dev/null
+++ b/releasenotes/notes/size-hint-ea2a264468e1fcb7.yaml
@@ -0,0 +1,4 @@
+---
+fixes:
+  - The "size" root device hint is now always converted to an integer for
+    consistency with IPA.