I am trying to earn a merit only for 5 days. Yes, I wrote 2 big posts (one disappeared) and thought they would be useful. I did not get any merit for them. That's mean I was doing something wrong.
Try listening to the advice you've already been given. Size is irrelevant. If a post disappeared and you didn't get a message saying it was deleted then that means the thread it was in was trashed. The spam megatheads you probably are posting in only exist for signature spammers to achieve their post count. Nobody reads them so nobody is going to give Merit to any post in them.
I came here to get advice from experienced users. But I got ridicule from some users, what is quite strange. It might be better not to ask anything. Sorry if I bother someone 
But why did you come here for advice? Why are you interested in getting Merit?
The only reason can be is that you want to gain rank so you can earn more from posting. This is a forum for Bitcoin and crypto enthusiasts do discuss the things they are interested in. Those people who come here for that purpose will have no problem getting Merit but equally, have little interest in getting it. You can discuss Bitcoin with a Jr. account as well as you can with a Legendary one. All the people that come here only to get paid to post and are obsessed with ranking up only get in the way and pollute the forum and that does bother a lot of us.