using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<Product> productList = new List<Product>() {

                new Product() {ProductNumber=1, ProductSize=5 },

                new Product() {ProductNumber=2, ProductSize=10 },

                new Product() {ProductNumber=3, ProductSize=3 },

            };

​

        var index = Enumerable.Range(0, productList.Count).FirstOrDefault(i => productList[i].ProductSize == productList.Max(x => x.ProductSize));

        Console.WriteLine("index={0}",index);

    }

}

class Product

{

    public int ProductNumber { get; set; }

    public int ProductSize { get; set; }

}