Go Fukkking Massive
Actually, you can't really make a bench too long, but you can make it too wide. Check out Rex Kruger on YT, one video about four common mistakes made when building your first bench and one about building an English Joiners bench.
I'm going to build one of his in the new year, it's going to be 1500mm long, 600 deep and about 850 tall. For context my current bench is 2m x 1m and I'm only using a square about 600 x 700 to work on. The rest has become stuff storage.