Currently, my settlement is about 30x18 squares, or roughly 24 expansions rounded up. I could wish for slightly more space and production but I don't know that I could do much better since I am limited to just 1 each of the Embassies. Also, I am waiting on KP as often as anything else, so it is probably at a fairly good balance.
My one piece of advice would be to produce Statutes using Portal Profits, not in the Senate. You will need the Amendments in the research tree, so it is a waste to use them to produce the Statutes.