The wallet should have you specify the fee PER KB so you don't need to know the txn size. Miners sort txns by fee/kb so a 2000 sat fee on a 2 KB txn is no better or worse than a 1000 sat on a 1KB txn.
My understanding is for every 1000 bytes, you should use 10,000sat, if you want to be in the next block.
Are you saying at 1000 bytes, you can use 1,000sat, and still be in the next block or within a few blocks?
I would assume blockchain.info does that as well but if it doesn't then that is a poor design.
Blockchain.info has two options for sending (not including Shared Coin).
(1) Quick Send - you enter the receiving address and amount of btc, and blockchain.info will broadcast from any address in your wallet and pay miners fee in proportion.
(2) Custom - you select your address(s) from your wallet , you enter the receiving address, you enter amount of btc, you enter miner fee, then blockchain.info broadcasts it.
My original question was how can I determine the my data size prior to broadcasting with blockchain.info.
But anyway, from my understanding, it does not do that within the custom setting and seems like it should provide that information.