From the whitepaper:

1)
New transactions are broadcast to all nodes.
2)
Each node collects new transactions into a block. 
3)
Each node works on finding a difficult proof-of-work for its block.
4)
When a node finds a proof-of-work, it broadcasts the block to all nodes.

So how does the the miner software work? I reckon transactions are flowing in continuously.
The time needed for the pow and the rate of incoming transactions determines the size of the block?