The question is, have there been in fact trams of 75 m length? It seems the world record for the longest tram belongs to the Combino in Budapest, of 54 meters.
But I think this isn't really about restricting the length, but more about planning the cars, considering economics and physics, and according to the features of each model. For instance, it makes no sense for two horses to be able to carry more than one car of reasonable size and weight (for 19th century trams). As to electric units, those also are usually not modular as trains are. The Combino is an exception, and indeed its length (and width) may vary greatly. But most modern trams come as units of a long articular car, not as separate cars you can assemble as you please. So perhaps it would make sense to unite some of these models (I'm not very familiar with the real life basis of the existing models in the pak T13 T24 etc.) into complete units, which may be of different lengths, 5, 10, 20, 30 meters etc. and have different speeds weights and capacities.
Another issue is how is length defined? How many meters is one tile? At least in simutrans experimental, this usually varies between 100-250 meters, so a tram should never be more than one tile. But then, scaling is far from perfect in simutrans. Still we need some sort of basis.