I did not know this before. Very, very useful. Shall use this method already bookmarked to test on my transactions for the next time.
Your client should be able to do this for you automatically.
Wait, you're saying the client can show you the tx size; can this be done before you make the transaction? I use Electrum for most of my transactions, but it's anyway of no concern since I adjust fees on per/byte and it calculates fees automatically. But I have recently startted to use counterparty (to see tokens on the Bitcoin blockchain) and while it also adjust fees automatically, I am unable to see the input/output size until after the spend is done.
Electrum makes it possible for me to choose which balances to use for my inputs, but not counterparty.
I'm really not sure about how counterparty works. But for Electrum, your transaction size is being shown to you when you click the "preview" button.