>>95991089
This made me have a dig through the old rulebooks looking for base size stuff cause I was sure I remembered them being mentioned.

3rd Edition Fantasy says "Models specifically intended for this game are supplied with the correct size base." Then carried over from 2nd edition each unit entry also says what base size each model should have so you can get it right if you buy a mini from somewhere else .

For Rogue Trader it just said "Most Citadel models are now sold with a separate plastic base of the correct dimensions for use with this game...As most combat takes place at a distance, and hand-to-hand combat is of only secondary significance, the actual dimensions of the base are not critical. A base should allow a model enough room to move and fight, without being so large that it looks ridiculous. The actual shape of the base itself is not important - and you can employ any round, hexagonal, or square bases available from Citadel."

This was still when you were recommended to have a Gamesmaster though, someone who could sort things out if a player was trying to gain an advantage from having a certain type of base, or no base.

Seems by 4th edition Fantasy, and into 2nd edition 40k not only was the Gamesmaster gone but anything in the rulebook about base sizes too.