The simplest way would be to start your `rxvt` using the `-sl`option: for example, `rxvt -sl 50000` to have a scrollback buffer of 50000 lines. The default is 1000 lines, at least on my Debian system.
Assuming that you are using a modern Unicode-compatible version of `rxvt`, the way to make this persistent would be to edit (or create) a file named `~/.Xresources` and add this line to it:
URxvt.saveLines: 50000
To make this setting effective, you'd need to logout & login again, or run this command in your current X session: `xrdb -merge ~/.Xresources`
This is a common mechanism for configuring classic X applications, known as X resources.
The thread you linked refers to increasing the scroll-back buffer on the _text-mode virtual console_ i.e. when the X11 GUI is not running, which requires kernel modification.