I have a dropdownlistfor:
@Html.DropDownListFor(model => model.Item.Item.Status, new SelectList(@Model.AllStatus, \"id\", \"Description\"),
SelectList ProductSizeList = new SelectList(_context.Sizes, "SizeId", "SizeName");
//after you create the selectList you have to create the default select list item
SelectListItem AllSize = new SelectListItem("All Size", "0");
// and after this you add this item to the begin of the selectlist
ViewData["SizeId"] = ProductSizeList.Prepend(AllSize);