If you're talking about making a monster occupy more cells, that isn't something that the game has already coded into it. All monsters take up 1 cell. I've never experimented with anything like that, so unless you feel like coding an entire new mechanic into the game... your closest bet is to increase the attack range on the monster in the monster database so that it gives the appearance of being "bigger".
You can potentially find more people who can help you with this at rathena or hercules's respective websites.