Prototype trains run typicall in the 70 to 80 car range with modern trains tending to be longer (modern unit and stack trains run in the 100-135 car/platform range). So pretty much any thing you have in your basement isn't "realistic". What you want to shoot for is what LOOKS acceptable to you. That could be be any size and has no bearing on what the prototype does ('cuse you ain't gonna be even close).
So pick what you think works for your space, design, era and operation. A lot of people in HO think a 40-50 car train is wonderful, I personally think its too long, but its all personal taste. My trains are designed to be in the 6-16 car range (1900-1905 era).