In current proof-of-work, there is an artificial time delay introduced because of difficulty. This allows the network to accept transactions that ultimately form the block. Vlad argues that once Ethereum moves to Casper, the time between 2 blocks can be 4 seconds or even sub-second.
On what is this time between block dependent in a PoS? How does the network know that everyone is using a synchronised clock? What will be the sizes of the blocks if there is a very small time gap between 2 blocks?