I found an April 7, 2017 post on Bitcoin Stack Exchange saying that
Over roughly the past two months, the mean transaction size was about 506 bytes, and the average count of transactions per block was 1,956. Current figures can be found here: Tradeblock Historical Data
I followed the link to see if I could find that sama data for Ethereum but could not locate it. Does anyone know the answer (or how / where to find it), and how it has evolved over time?