31MB is sufficient for roughly 2400 transactions (or 4tps). The exact size of an individual transaction depends on the number of in/out addresses. Over the last 1000 blocks, block size has averaged 17KB with 40 transactions putting average transaction at ~420 bytes each. The protocol can support larger block sizes. – DeathAndTaxes – 2011-10-26T22:33:43.643

"Each block needs to have at least one transaction - one paying the miner the reward for mining the block.". How does that work once mining has stopped? Of course, that is still a while off... – Thilo – 2011-10-28T08:49:18.607

When the mining stops, there won't be blocks to generate;). Mining won't stop, due to the transaction fees and what nots. I'm guessing that over 150 years from now, when new coins won't be created any more, and on an odd block that won't have any fees, the first transaction will be sending 0 coins to yourself. – ThePiachu – 2011-10-28T09:54:32.927

1@Thilo Mining (better term is hashing) never stops. Hashing blocks into the block chain is required part of Bitcoin network. The "end game" goal of Bitcoin is for transaction fees to pay for the cost of the network. Currently fees are insufficient so miners get a block reward which is essentially a subsidy. Those subsidies will slowly decline and be replaced with transaction fees. There is always a coinbase transaction (reward + fees). As ThePiachu pointed out if the block reward & fees are 0 that transactions still exists and simply be 0 BTC sent to the miner. – DeathAndTaxes – 2011-10-28T18:10:31.397