When growing up my Grandparents ordered coal for heating purposes in winter. They had big piles of it when the heating period started. There where huge chunks of maybe 50cm length and 30cm width. I guesstimate the whole pile to be around 10m^3. But keep in mind it’s not the most reliable source since this dates 30+ years back and the dimensions have been seen with a little kids eyes. It may be less.

My house I live in today is 100+ years old. There are still some pieces of coal in my basement.