How big were the vacuum tubes used in early computers?

Quora User's answer is spot on as to what the ENIAC used, but he neglected to provide the physical dimensions. read more

They were relatively large, octal based tubes, the 6SN7 and 6SA7 were used the ENIAC. They had the advantage of being a double triode, good for storing one bit as a flip-flop, and a multi-grid tube, good for making an 2-input gate. Later on in the 1950's they used smaller tubes, the 6J6 and 12AU7 were popular as smaller versions of the 6SN7. read more

Because vacuum tubes were the best available technology for building electronic logic circuits at that time. Solid state devices at the time were unreliable. Once the transistor was invented and developed into a reliable product, the vacuum tube's days were numbered. Earlier alternatives to the Vacuum tube were electrical switches and relays. read more

The type of tube used in early computers was called a triode and was invented by Lee De Forest in 1906. It consists of a cathode and a plate, separated by a control grid, suspended in a glass vacuum tube. read more