As referenced in the following KB http://support.microsoft.com/kb/2669081 for previous versions of Exchange you were able to increase the size limit of the Junk Email lists.  By adding the registry setting, Max Extended Rule Sizeand restarting the information store.

Is this resolution still applicable for Exchange 2013 running on Windows Server 2012 R2?

Hi 

yes this solution is applicable for Exchange 2013 as well as far my knowledge is concerned 

Hi,

I have tried this in 2013 and it doesn't work. Please could you confirm if this is the case and if there is any other work around?

You are correct.  This does NOT work in Exchange 2013.  We ended up opening a case with Microsoft regarding this.  Potential workarounds would depend on exactly what you are trying to accomplish.  That being said, the actual registry change that should increase this limit does not work and there are no plans for Microsoft to fix this bug.