Bitcoin Block Size - What are the rules? What is the maximum size a block can be? 
Is there a minimum size?
Can you have a block with 0 transactions included?
The current maximum block size is 1MB (but it could be increased in the future with a protocol change), there is no minimum size per se, but the block needs to have all its components to be valid (check Protocol Specification). Each block needs to have at least one transaction - one paying the miner the reward for mining the block.