You need to go into your server files (Files -> FTP File access on Multicraft) and open the file bukkit.yml.
The values that you need to edit are in this section:
Edit the values as you see fit, then save the file and restart the server.
Note: The more monsters or animals you have on the server the more of a performance impact you will have. This means that your server will start lagging if you have too many monsters or animals.