How to pass dropdownbox select value to a controller in asp.net mvc using Ajax?

Question

I have the following DropDownListFor generated by razor:

@Html.DropDownListFor(m => m.SelectedSize, Model.AvailableSizes, "Select Size", new { @class = "form-control input-sm", @id ="availablesize" })

I want to pass the selected value of DropDownList to a controller ViewResult method:

public ViewResult Index(Cart cart, string returnUrl)
        {
            return View(new CartIndexViewModel
            {
                ReturnUrl = returnUrl,
                Cart = cart,
            });
        }

Help please!