Query request size information
Lots of new users to Cassandra struggle with understanding a given request's size, they either incorrectly equate their source information size as being equivalent (text file or other database). So my suggestions are as follows
Provide a metrics summary from the session object where we store average request size over say the last 5 minutes. This will probably be suitable for the proof of concept or getting to design your data model phase. Likewise it'll help DBAs diagnose client side problems more readily.
On a given statement give the ability to retrieve 'parameter size' this would not give them completely accurate numbers, but it maybe enough to help aide in data modeling as it could become the unit of measure for their performance numbers
Log WARN requests over a given size, and make that given size
numbers 1 and 3 would be aided by implementation of query listeners discussed for Python Extension Support by Jon Haddad already in internal documents.