Another incentive to grow wide instead of tall, because its faster and cheaper to expand than develop.
Only as far as you need to grow to become a kingdom. Most players start as a kingdom anyway, or intend to form one fairly quickly. (The main exception would be playing as Austria or something, where you're prevented from ascending by HRE politics rather than not being big enough.)
Also, growing wide takes a lot of diplomat man-hours, whereas growing tall doesn't really need diplomats.