public int Id {get; set;}
    public int S {get; set;}
    public int M {get; set;}
    public int L {get; set;}
    public int ProductId {get;set;}
    public Product Product {get; set;}

var product = db.Products.FirstOrDefault(p => p.Id == productId);
            var availableSizes = from s in db.ProductSizes where (s.Id == productId) select s;
            List<string> Sizes = new List<string>();
            foreach (var s in availableSizes)
            {
                if(s.S == true)
                {
                    Sizes.Add("S");
                }
                if(s.M == true)
                {
                    Sizes.Add("M");
                }
                if (s.L == true)
                {
                    Sizes.Add("L");
                }
                if (s.Xl == true)
                {
                    Sizes.Add("XL");
                }
                if (s.Xxl == true)
                {
                    Sizes.Add("XXL");
                }
            }
            return Sizes;