There is no maximum limit stipulated on a page size in Confluence. However, if a page larger then 2Mb is created, an user can experience two problems:
Rendering of a page is slow
Understandably, rendering of a large page is going to take excessive time. Don't try to put the entire content of a book into one page, split it into pages/chapters.
It is easier to manage and fast to render too.
HTTP POST error when Tomcat 5 is used
This error manifests that a page is far too large, larger then 2 MB.
Apache Tomcat 5, by default, sets the maximum size of acceptable HTTP POST request to 2MB.
You can reconfigure Tomcat to accept larger requests. This can be done by increasing the allowable limit or just simply disabling this functionality.
The file you need to edit is
<Tomcat-Dir>/server.xml. Set the Tomcat configuration parameter maxPostSize for the HTTPConnector to a larger value (in bytes)
to increase the limit. Setting it to 0 will disable the size check.
See the Tomcat Configuration Reference for more information.
maxPostSize - "The maximum size in bytes of the POST which will be handled by the container FORM URL parameter parsing. The feature can be disabled by setting this attribute
to a value inferior or equal to 0. If not specified, this attribute is set to 2097152 (2 megabytes)."