You can do this using UI script Checkout this article for detailed steps:

Client side utility using script UI script

Hi there,

Here is a Script Include which returns true/false based on your specs (make sure 'Client Callable' is ticked on the same):

var CPcheckDescriptionLength = Class.create();
CPcheckDescriptionLength.prototype = Object.extendsObject(AbstractAjaxProcessor, {
	
	isLengthSatisified: function(){
		// We can store the value 251 in a system property, or can just set it statically here:
		var minimum = gs.getProperty('STORE_THE_VALUE_IN_PROPERTY') || 251;
		
		// Check if our short desc is less than minimum, return true/false
		return this.getParameter('sysparm_sd') < minimum;
	},

    type: 'CPcheckDescriptionLength'
});

Then from your client script call it like this:

function onLoad() {
   //Type appropriate comment here, and begin script below
	
	var ga = new GlideAjax('CPcheckDescriptionLength'); // The name of the Script Include
	ga.addParam('sysparm_name', 'isLengthSatisified'); // The name of the method we want to use in the SI
	ga.addParam('sysparm_sd', g_form.getValue('short_description')); // Pass your short description to the param we define in the SI
	ga.getXMLAnswer(function(response){ // Callback function to use the 'response' from our SI
		jslog(response); // Will be true or false
	});
   
}

Thanks Cris!  I am trying to use this in the onChange script that I created to compare the short description and description.  I am getting a javascript error on the portal record producer though.

Here is the onChange client script.  Also, how to I add an alert popup if the description is under 251 characters?

function onChange(control, oldValue, newValue, isLoading, isTemplate) {


    if (isLoading || newValue === '') {
          return;
    }

    //Verify that Short Description and Description are not the same

        var title = g_form.getValue('short_description');
        if(title == newValue)
        {
                  alert('Description cannot be the same as the short description.  Please enter additional information.');
                  g_form.clearValue('description');
        } 
	
	//Verify that Description is at least 250 characters

		var ga = new GlideAjax('CPcheckDescriptionLength'); //The name of the Script Include
		ga.addParam('sysparm_name', 'isLengthSatisified'); //The name of the method we want to use in the SI
		ga.addParam('sysparm_sd', g_form.getValue('description')); //Pass your description to the param we define in the SI
		ga.getXMLAnswer(function(response){ //Callback function to use the 'response' from our SI
			jslog(response); //Will be true or false
		});
 
}

I tried this with the onLoad script and cannot get it to work.  I get a javascript error when the record producer loads

Have you considered using a variable set for this?

The variable set would only contain a single field - Description, and the client script.