There is no reason to not use both, the 1st culture I always grab tradition, no matter what I plan, hard to say no to +3 culture. Now to answer your question, Small to me ie Tall empires are 1-6 cities. Wide are over 6, after 6 the culture cost is crazy to me. Again try your own test, I will rarley do more then 4 cities now, 3 asap, and I will settle 4th if no iron and some is close, if no iron i stay at 3.