I have edited my config to include larger sizes, and reloaded the plugin, but I can't get more than 3 pages.

Wondering what I am doing wrong.

You will have to grant the permissions for the new sizes. For example, backpacks.size.192 (replace 192 with whatever number you put in the config). If the permission is applied successfully, but the backpack size does not increase, then you may have dynamic size enabled, which supersedes the normal size permissions. If so, you can revoke any permissions associated with dynamic size or disable that feature in the config.