I installed MEPIS, and something went wrong with the GRUB installation; it was saying Error 18. I tried reinstalling GRUB from the MEPIS liveCD, but the problem kept recurring. And what does Error 18 mean?

This LQ wiki article explains Error 18 and how to solve it. (Remember, Google is your friend! )

The maximum size issue should not be an issue with newer computers.

One of the most common issues in installing GRUB is getting it pointed at the /boot partition where it will find the config file (menu.lst), stage files, etc.. This gets "hard-coded" into the code that goes in the MBR.

When installing GRUB (from anywhere), the "root (hdX,Y)" command must point to the location where /boot will be when the machine is booted normally. This may be different than the configuration seen when running from a LiveCD.

I concur with pixellany's view and suggest the OP to try the terminal method as follow

1. Boot up Mepis Live CD but instead of choosing a system to boot press "Esc" immdiately. You will be warned that you are leaving the graphic screen. Select Ok and now you are in text mode of Grub. You then press the "c" key to talk to Grub directly. No middleman involved everything from the horse's mouth now.

2. You ask Grub to tell you which partition has its menu.lst by command
Grub should reply with something like "(hd0,0)"

3. You restore Grub by terminal command
Adjust the red bit to suit your own circunstance.

4. You now boot up Mepis by commands
Report any error message if the above fails.