This is probably (most definitely) going to be seen as a stupid question... but how do I calculate my transaction fees? My hardware wallet is saying that transferring 0.1 BTC has a transaction fee of 0.0084396 BTC (about $24.77 at the time of posting this). I've heard of https://bitcoinfees.21.co/ and it says: 
The fastest and cheapest transaction fee is currently 390 satoshis/byte, shown in green at the top.
For the median transaction size of 226 bytes, this results in a fee of 88,140 satoshis.

Do I need to know my transaction size in bytes in order to calculate my transaction fees? If so, how do I find that out? Also, I mine with NiceHash ~.016 BTC per week? Could that be why my transaction fees are so high? Is there a way to get it to be lower?