--- a/dom/media/TextTrackCue.cpp
+++ b/dom/media/TextTrackCue.cpp
@@ -30,17 +30,17 @@ StaticRefPtr<nsIWebVTTParserWrapper> Tex
 
 // Set cue setting defaults based on step 19 & seq.
 // in http://dev.w3.org/html5/webvtt/#parsing
 void
 TextTrackCue::SetDefaultCueSettings()
 {
   mPosition = 50;
   mPositionAlign = PositionAlignSetting::Center;
-  mSize = 100;
+  mSize = 100.0;
   mPauseOnExit = false;
   mSnapToLines = true;
   mLineIsAutoKeyword = true;
   mAlign = AlignSetting::Middle;
   mLineAlign = LineAlignSetting::Start;
   mVertical = DirectionSetting::_empty;
   mActive = false;
 }

--- a/dom/media/TextTrackCue.h
+++ b/dom/media/TextTrackCue.h
@@ -220,28 +220,28 @@ public:
     if (mPositionAlign == aPositionAlign) {
       return;
     }
 
     mReset = true;
     mPositionAlign = aPositionAlign;
   }
 
-  int32_t Size() const
+  double Size() const
   {
     return mSize;
   }
 
-  void SetSize(int32_t aSize, ErrorResult& aRv)
+  void SetSize(double aSize, ErrorResult& aRv)
   {
     if (mSize == aSize) {
       return;
     }
 
-    if (aSize < 0 || aSize > 100) {
+    if (aSize < 0.0 || aSize > 100.0) {
       aRv.Throw(NS_ERROR_DOM_INDEX_SIZE_ERR);
       return;
     }
 
     mReset = true;
     mSize = aSize;
   }
 
@@ -357,17 +357,17 @@ private:
   double mStartTime;
   double mEndTime;
 
   RefPtr<TextTrack> mTrack;
   RefPtr<HTMLTrackElement> mTrackElement;
   nsString mId;
   int32_t mPosition;
   PositionAlignSetting mPositionAlign;
-  int32_t mSize;
+  double mSize;
   bool mPauseOnExit;
   bool mSnapToLines;
   RefPtr<TextTrackRegion> mRegion;
   DirectionSetting mVertical;
   bool mLineIsAutoKeyword;
   double mLine;
   AlignSetting mAlign;
   LineAlignSetting mLineAlign;

--- a/dom/webidl/VTTCue.webidl
+++ b/dom/webidl/VTTCue.webidl
@@ -46,17 +46,17 @@ interface VTTCue : TextTrackCue {
   attribute (double or AutoKeyword) line;
   [SetterThrows]
   attribute LineAlignSetting lineAlign;
   [SetterThrows]
   attribute long position;
   [SetterThrows]
   attribute PositionAlignSetting positionAlign;
   [SetterThrows]
-  attribute long size;
+  attribute double size;
   attribute AlignSetting align;
   attribute DOMString text;
   DocumentFragment getCueAsHTML();
 };
 
 // Mozilla extensions.
 partial interface VTTCue {
   [ChromeOnly]