How to get width and height of a CustomPaint

Question

I am trying to make this view with CustomPainter and GestureDetector onPan, So it is a normal Scribble Screen.

GestureDetector(
        onPanStart: onPanStart,
        onPanUpdate: onPanUpdate,
        onPanEnd: onPanEnd,
        child: RepaintBoundary(
          child: Container(
            color: Colors.transparent,
            alignment: Alignment.topLeft,
            child: StreamBuilder<DrawnLine>(
              stream: currentLineStreamController.stream,
              builder: (context, snapshot) {
                return CustomPaint(
                  painter: Sketcher(
                      normalise: false,
                      lines: [snapshot.data],
                      color: Colors.black,
                      strokeWidth: 5.0),
                );
              },
            ),
          ),
        ),
      )

  void onPanUpdate(DragUpdateDetails details) {
    if (!checkBounds(context.size.topLeft(details.localPosition))) return;

    RenderBox box = context.findRenderObject();
    Offset point = box.globalToLocal(details.globalPosition);
    List<Offset> path = List.from(line.path)..add(point);
    line = DrawnLine(path: path);
    currentLineStreamController.add(line);
  }

Problem is that i need to bring this scribble paint to another screen as a draggable widget with keeping its shape and dimension.

I am getting its relative offset of the line by

path.forEach((element) {
      element = Offset(element.dx / size.width, element.dy / size.height);
      tempPath.add(element);
    });

Now i have offset which i can acurratly position the painter in given size. But I don't know how to get width and height of a drawing.

context.size gives me the whole Container dimemsions but is it possible to only get dimensions of the paint that i drew with gesturedetector?