is there a limit to a page in “pages”?

The post_content field is of the longtext type. Quote from: https://www.phpfreaks.com/mysqlref/34.php

A TEXT column with a maximum length of 4,294,967,295 or 4GB (2^32 − 1) characters. Up to MySQL 3.23, the client/server protocol and MyISAM tables had a limit of 16MB per communication packet / table row. From MySQL 4.0, the maximum allowed length of LONGTEXT columns depends on the configured maximum packet size in the client/server protocol and available memory.

As for the page_id=6 look at Using_Permalinks and Administration > Options > Permalinks.

I’m having the same problem, it seems that up to a certain length the pages work fine, but go over that length and wp truncates the page data to a very small portion of what was originally pasted in.