Input*180 + output*34 + 10 plus or minus input
Say, input is 2 and output is to 3 address. The size is-
2*180 + 3*34 + 10 + or - 2 = 470/456 bytes.
There's a much easier solution: use
https://coinb.in/#fees, and in a few clicks you get the size including a recommended fee for a fast transaction. Note that most transactions don't need to be
fast.
Thank you but it was an initiative of learning the process how it actually get done.
we do see a +2 or -2 resulting into 470/456 bytes.
It should be 474/470 bytes. My initial calculation was wrong. I guess you got it now.