I am aware that for comments we have maximum of 600 characters to type and by curiosity I just happened to type some random words in a comment box. I expected that when comment exceeded 600 characters I will not be able to type but instead it showed - 'you exceeded X characters'. I am just curious but afraid to do this - what happens if I post a comment with more than 600 characters?
In a way literally nothing happens. You click "Add Comment" and the situation stays unchanged. The "too long by" flickers briefly, but that's it.
As explained in a comment by amWhy there is a check on submission.
I dimly recall that in the very early days this was handled much less gracefully, and the text in the comment box got cleared instead.
What will happen when I write a comment longer than 600 words?
As such nothing happens. When you will write the comment longer than 600 words and click on add comment it will display a message "Too long by $x$ characters" where $x$ represents total characters minus 600. Here is a refrence what happens when I enter something longer than 600 characters-
What actually happens is that as soon as the system finds the number of characters to be more than 600 it says too long by some k characters. If you click on Add Comment as such you won't get any message but your comment won't be added.
What to do if I want to post a comment longer than 600 characters
If you feel that that the comment is much longer than 600 characters and can be helpful to answer the question just post it as an answer but remember before posting that do write something like "Just an extended comments". If you feel like you are giving opinion and will not much be helpful in answering the question you can definitely type first 580-590 characters in 1 comments and then in next comment put the remaining words.
Ok I understood this but why the limit on comments
As stated by User Guffa "The limit on the comment length is pretty reasonable. If you reach the limit, it's usually because you are using the site in a way that was not intended. The commenting function simply isn't suitable for something like a long discussion, so if you try to use it for that it will naturally feel very limited."