Too Long; Didn't Read
The block size is the biggest constraint to the development of the Ethereum network. The exact limit varies, but currently it is about 10 million gas. Each operation has a different gas cost, but at the end you need to remember that reading one data element from storage is 200 gas. The key to code smart contracts with the maximum utility is to be very careful with the data structures that we use, to know the limits to computation inherent to block gas limits, and to break down work into separate calls when everything else fails.