Well Eduffield just put up a decent idea in the Darkcoin blog on the issue


The Bitcoin developers and community are talking about expanding the maximum blockchain size to 20MB. Currently the Bitcoin network can handle 7 transactions per second. This will allow their currency to 140 transactions per second, but will also result in up to 1TB per year in blockchain storage (until they successfully implement pruning).

Eventually Gavin Andresen sees blocks getting to 100MB+, with tens of thousands of transactions per block. See http://gavintech.blogspot.com/2015/01/twenty-megabytes-testing-results.html and https://docs.google.com/spreadsheets/d/1CuOEM9uwO5w-RwWGCCZpVGVFwhHHHegxJZqTP5KyapI/edit#gid=0.