Check the size of PlayerPrefs from web player

Is there any way to check the current size of PlayerPrefs from the webplayer? I'm currently working on a web based game. The idea is to store some data locally to avoid frequent calls to the server. I can see some situations where multiple users on the same computer can overload the capacity of PlayerPrefs. I realise I could just catch the error when it occurs, but I would prefer to know what space is available before the limit is reached.