controller.php

public function postVendorCreateItems ()
        {
            
            $input     = Input::all();
            $validator = Validator::make($input, 
              [ 'item_name'     => 'max:50',
                'normalprice'   => 'numeric',
                'karmaprice'    => 'numeric',
                'asin'          => 'max:50',
                'sizes'         => 'max:50',
              ]);

            if($validator->passes())
            {

                $items                 = new Item();
                $items->brand_id       = $input['brand_id'];
                $items->name           = $input['item_name'];
                $items->normalprice    = $input['normalprice'];
                $items->karmaprice     = $input['karmaprice'];
                $items->asin           = $input['asin'];
                $items->sizes          = $input['sizes'];
                $items->save();

                return Redirect::route('account-vendor-index')
                               ->with('global', 'You have successfully created an item.');
            }

            return Redirect::route('account-vendor-index')
                           ->withErrors($validator)
                           ->with('global', 'Your could not be created.');
        }

view.php

<div class="form-group">
                    <label class="col-xs-3 control-label">Sizes</label>
                    <div class="col-xs-9">
                        <select id="selectSizes" multiple="multiple" class="form-control selectSizes" name="sizes[]">
                            <option value="XS (0-2)">XS (0-2)</option>
                            <option value="S (4-6)">S (4-6)</option>
                            <option value="M (8-10)">M (8-10)</option>
                            <option value="L (12-14)">L (12-14)</option>
                            <option value="XL (16-18)">XL (16-18)</option>
                            <option value="XXL (20-22)">XXL (20-22)</option>
                        </select>
                    </div>
                </div>