rule plan string too big?

I am getting the following error from a CREATE RULE statement:

	ERROR:  DefineQueryRewrite: rule plan string too big.
 
This statement is only 377 characters in length.  What are the limits
to the size of rules?  Are they changable at compile time?  Is this
the same as a "normal" tuple (i.e., 8K)?  If so, how do 377 characters
expand to over 8K?

Thanks for your help.

Cheers,
Brook