In terms of search engine more accurately matching the material to the title as it is told, the better.
You are in a tutorial usually write fairly small headlines - and it looks nice)
but I am a lawyer, and a typical header looks like this - "the basic rules of the marriage contract in New Zealand law."
So I wanted to ask - How many letters can contain the article title?
Can I make the header is printed in two lines - not to make the menu half the size of the monitor screen
It all really depends on the theme you are using. There is no hard-limit on the number of characters. You just need to ensure the theme + CSS allows long titles to wrap properly and look ok. It’s not really a Grav thing, its a just a web/html thing.