can you recommend a wallet where i can see the byte size before i hit deposit ?
Unfortunately it is basically impossible to predict the size in bytes of a transaction before you actually send it. The best you can do is to find out what the current recommended fee rate is and set that as the fee rate for your transaction before you send. Even better if you can see what the fee rate your wallet is using at the time so you know whether it is high enough.
I recommend that you use Electrum, although they have an issue where the fee rate has a maximum of 50 satoshis/byte. You can fix this by just setting a fixed fee rate in the settings to whatever the recommended rate is. Do this before each transaction.
If you use a wallet that has "coin control" features (such as Bitcoin Core), then you can choose which inputs you want to use, and which outputs you want to use. Doing that, you can get a reasonably good estimate of the size of your transaction.
A reasonable estimate would be:
- 10 bytes of general transaction overhead
- 149 bytes per input
- 34 bytes per output
There is a small variation in the size of inputs (they can actually be anywhere between 147 and 149 bytes) so the actual size can be a bit smaller than this estimate, but using this you can be relatively certain that the transaction won't be bigger than estimated and will be relatively close.
So, for example, your transaction with 4 inputs and 2 outputs would be:
10 + (149*4) + (34*2) = 674
10 + (147*4) + (34*2) = 666
So your size was between 666 and 674 bytes. As you mentioned, it was 670 bytes, but if you had used 674 when calculating you'd have been close enough.
