A bit of quick maths reveals to me that one of my smaller worlds is a fair bit more than 3 million blocks. Seeing as the height of each cell is, I believe, 128, and the width and depth are 64, just six cells reaches over 3 million, and I usually end up with several dozen cells exploring (most of my worlds extend out very far in every direction as I look for cool natural formations to mess around with, ending up with 20MB worlds).