WordPress Plugin Help database storing / retrieving value

I'm a newbie programmer and I am trying to develop a wordpress plugin.

I have created an admin panel and set the text field and submit button. My problem is adding and retrieving the value from the data base.

Here is the Form:

function mms_form () {
	
	$default_value = get_option('default_size');
	global $default_value;
	
	if(empty($default_size)) {
		add_option('default_size',50,'short url','');
		}
	else {
	$default_size = get_option('default_size');
	}

	?>
    <form method="post" style="width:60%; margin: 5% auto;">
    	<label for="chunk_size">URL Size:
        	<input type="text" name="chunk_size" value="<?=$default_size ?>" />
        </label><br />
        <input type="submit" name="submit" value="Submit" />
	</form>
    <?php
}

Code (markup):

Here is the options update:

function update_mms_options() {
	$ok = false;
	
	//input validation
	if($_REQUEST['chunk_size']) {
		update_option('default_size', $_REQUEST['chunk_size']);
		$ok = true;
	}
	
	if($ok) {
		?><div id="message" class="updated fade">
        	<p>Settings Updated</p>
          </div><?php
	}
	else {
		?><div id="message" class="error fade">
        	<p>Captain, Captain an error has occured</p>
          </div><?php
	}
} // update_mms_options

Code (markup):

The text field isn't sending nor retriving the #default_size value.

Can anyone help me out here?

thanks