Large Model Trains Is There A Standard Size For Model Trains?

Is there a standard size for model trains? - large model trains

Sine I was a little boy who wanted to collect model trains. Recently my gf bought me a model for my bday. I have seen different sizes in hardware stores and asked me if there is a "popular" in size. I do not know how big (no offense to those who do not will not) "Tiny" trains ... Is the "NH", a common size?

My intention is to build a railway to see monumentous "cities" in exhibitions. I do not want to spend years collecting pieces, it can be difficult to follow.