Size of a context-free grammar
What is your intended application.  The article you cited does give a definition of size.  A production rule

T -> R

has size |R|+1, where |R| is the number of symbols in R.  A more precise definition can be found on page 9 of this document.

This notion of size is meant to compare different grammars that produce the same language.  In particular, in the article you cited, the notion of size is used to see that under grammar transformations, the size of the grammar can explode.