How to access key/value of the array in Laravel 4?

The short answer is you can't, or at least not in the way you're expecting. When an item is selected and posted back to you, all that is sent back is the value of the <option> element, not it's content.

Note that:

{{ Form::select('size', array('L' => 'Large', 'S' => 'Small'), 'S') }}

produces HTML a little like this:

<select name="size">
    <option value="L">Large</option>
    <option selected value="S">Small</option>
</select>

But the key-value pairing between "L" and "Large" etc. is now lost. It's used to create the HTML markup, but nothing more - it's not stored anywhere for you to access it directly through Laravel.

When your form is submitted and posted to wherever it's meant to be posted, your browser only sends the value, in this case "L". Laravel has no idea that "L" means "Large". You will need to have this information stored elsewhere, for example:

$sizes = array(
    'L' => 'Large',
    'S' => 'Small'
);

Then you could use (provided $sizes is passed to your View):

{{ Form::select('size', $sizes, 'S') }}

and retrive the value with $sizes[$shirt->size].

If you want to be extra careful (as someone could edit the HTML to submit a value different from "L" or "S"), you can do something like

$size = (array_key_exists($shirt->size, $sizes) ? $sizes[$shirt->size] : '');

A second idea, as it looks like you have a Shirt model, would be to add the following to that class:

public $sizes = array('L' => 'Large', 'S' => 'Small');

public function getSizeAttribute($v) {
    return (array_key_exists($v, $this->sizes) ? $this->sizes[$v] : '');
}

Then $shirt->size will do exactly what you want and spit out "Large".