A Firkin is a quarter size barrell holding 9 Imperial Gallons
Thats about 40 litres or Kilos, so if you allow for the weight of the barrel, its moveable by a couple of delivery (or dray) men, so a popular size for pubs. The pin, at half the size/weight could be carried (home)
Barrel Name Size (Gallons)
Pin or Polypin 4.5
Firkin 9
Kilderkin 18
Barrel 36
Hogshead 54
Butt 108
Tun 216
A firkin is only 7 1/2 gallons in America, but 70 gallons of wine, and I think there is a metric firkin too