What brings merit is quality post but the quality of your post is subjective which means it depends on the merit source spread merit based on what he /she considers as quality post.
Indeed. Sometimes the quality can be quite subjective, each person can have a different perception about the quality. It is the same as the information we share on our post, some people may think it can be useful but other may think it is a common thing.
We don't only rely on the merit source to get merits. Each person who has sMerit, they can give us merits as well. However, they sometimes can be very generous but they can also be very stingy.
A quality post can either be lengthy or short but it's more about the information that it contains. Sometimes your post can be very informative but it has been treated or it's known to the people already hence they do not consider it anymore.
Yes. The quality doesn't always depend on the length of the post. A short post can be a quality post if it is an informative or useful post. However, it is true that whether it is informative or useful can be subjective. It depends on how the person who view the post.
But perhaps you want to earn some merit for good post then you have to look at how people who have been earning some good merit does it and then if you follow the steps it certainly bring you some merit too.
It is not a must to follow other people's ways. But makes sure our posts fulfil the minimum standard of quality posts.
At least, it should be : informative, constructive, easy to understand, and follow a correct writing method.