To reasonably use this I need to know a size of a transaction. But I can get it only after I press the button "Send", so after a fee is already set and the transaction is out of my control.
No, you don't need to know the size of the transaction. B.info offers you the ability to specify the fee "rate" in sats/byte... as long as the rate you input is similar to the rates specified by either
https://bitcoinfees.earn.com/ or
https://btc.com/stats/unconfirmed-tx then, your transaction should not get stuck, as the total fee will be calculated correctly for your transaction size.
https://i.imgur.com/qIqEy2m.pngSuper! Thank you very much.
