Flash Painting AS3 (problem with paint board)

Hi friends
My Flash Painting Project has a a few problems . sorry about my language…
when i draw in my board the Brush has exceed the board … why??? . also when i clear my board it exceed from original size and any time i press my Eraser Tool the board has exceed Alternatively in large and larger size of stage.
i know a little bit as3 please help me … thanks