How to limit characters in system.xml for textarea in magento

Add the following to the <orderPlaceMessage> element:

<validate>validate-length maximum-length-150</validate>

This will add these validation CSS classes used by prototype.js. If you enter a longer value, you will see this generic validation message:

So it is a good idea to add a comment as well to let the user know what this range is:

<comment>Maximum length: 150 characters</comment>

If the limit is important for data integrity, you should also add server side validatation, using a backend model. Digital Pianism already linked a tutorial for this: http://alanstorm.com/magento_system_config_validation

And if you want to know more about the various options in system.xml, there is: http://alanstorm.com/magento_system_configuration_in_depth_tutorial