You cannot know the size in bytes of a transaction without knowing the inputs to the transaction. Just knowing the amount you want to transfer and just the output does not tell you anything.
sendCoins() method and setting
feePerKb in your
SendRequest should create a transaction that pays the specified fee rate. If you want to get the transaction details after you send, the
SendResult that you get has a field named
tx which is a
Transaction object. You can then get the necessary information from there.
If you want to know the size before you send, then you need to craft the transaction yourself as
sendCoins() will automatically create and send the transaction for you.