The size of the blockchain has nothing to do with Bitcoin adoption. Blockchain's size increases with every block added to the blockchain, which usually happens every ten minutes. And each block being of 1 MB in size would increase the size of the blockchain by 1 MB.
Right. I'm very new to this. I wasn't thinking about the size of the chain so much as the size of the blocks. What if there's tens of thousands, or hundreds of thousands of new transactions per block (as there will be when everyone starts using bitcoin on a regular basis!). All those transactions in one block of 1MB?