MSSQL 的方法

public int GetColumnMaxLength(string tableName, string columnName)
{
    int result = -1;
    using(var conn = new SqlConnection("ConnectionString"))
    {
        conn.Open();
        var cmd = new SqlCommand("SELECT * FROM "+tableName+" WHERE 1 = 0", conn);
        var reader = cmd.ExecuteReader();
        var schema = reader.GetSchemaTable();
        result = schema.AsEnumerable()
                         .Single(s => s.Field<string>("ColumnName") == "Column")
                         .Field<int>("ColumnSize");
        conn.Close();
    }
    return result;
}

你是EF Core的話，你可以在Entity設定欄位狀態

https://docs.microsoft.com/zh-tw/ef/core/modeling/entity-properties?tabs=data-annotations%2Cwithout-nrt

或是自己寫簡單的驗證

public class Program
    {  
        static async Task Main(string[] args)
        {
            List<Student> student = new List<Student>
            {
                new Student { Name = "一" },
                new Student { Name = "一二" },
                new Student { Name = "一二三" },
                new Student { Name = "一二三四" },
                new Student { Name = "一二三四五" }
            };

            Console.WriteLine(string.Join(",", student.Select(x=>x.Name)));

            Console.ReadKey();
        }
        public class Student
        {
            private string _name;

            public string Name
            {
                get
                {
                    if (_name.Length <= 3)
                        return _name;
                    return "溢位";
                }
                set { _name = value; }
            }

        }

    }