Is there a limit on the block size in tezos?
If so, is it expressed in a maximum number of transactions? Or maximum byte size? Or something else?
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In Tezos Alpha the Transaction List size is limited:
Tezos does not download blocks all at once, but rather considers headers and various lists of operations separately. In Tezos.alpha, a maximum size in bytes is applied to the list of transactions MAX_TRANSACTION_LIST_SIZE = 500kB (that’s 5MB every 10 minutes at most).
Other lists of operations (endorsements, denunciations, reveals) are limited in terms of number of operations (though the defensive programming style also puts limits on the size of operations it expects). 
About year ago they had a target of 1 minute between blocks and no size limit set so far:
We're targeting 1 minute between blocks. We haven't set a blocksize yet, but we'll her on the side of caution at first. It's better to raise the blocksize through the amendment mechanism once we are confident in the network's throughput.