How to calculate transaction size (kb)
Count the number of inputs in the transaction.
That is the approximate size of the transaction in kB.
Counting the number of inputs / output needs a trasaction to be sent right?
Nope, That is not what is mean, I mean knowing the total size of kb before sending transaction
Wallets don't show size before sending transaction as they choose fees automatically. You can rither create transaction by yourself(which I don't think you will) or calculate an estimate fee by following DannyHamilton's guide.
Count the number of outputs in the transaction. Multiply that number by 34.
Compute the sum of those two numbers and add 10.
Divide the result by 1000.
That is the approximate size of the transaction in kB.
-snip-