Here's a great training on adding the "read more" tag into a post: Using The More Button To Add A Read More Let me know if that works for you!

In the back office depending on your theme, go to appearance=> settings and change it.

Do not know what U mean.

Very well explained by nathaniell!

I would say like Nathaniel on this... anything above 1000 words. You have a template in the sitecontent section of WA that help you do this: https://my.wealthyaffiliate.com/content/templates

1000 is a good place to start, but you should write enough so that the subject is covered completely. You should write enough so that a person doesn't feel like they have to leave your website to get more information. If you can't fit all the info into one targeted post, you can write a second post, then link post 1 to post 2. That's called "internal linking"! Internal Linking Strategies for Rankings

I believe the training recommends about 1,500 words. I shoot for 1,000 minimum. I do believe that quality is better than quantity though.