Geriau naudok KeyUp eventa.
Taip pat ka daryti jei max dydis gali buti didesnis nei 9 ? :)
WinFormoje keisti contorlo propercius geriau ish to threado kuris ta
controla sukure .... Todel naudok kazka tokio:

        int maxSize = 5;
        private void textBox1_KeyUp(object sender, KeyEventArgs e)
        {
            int myInt = 0;
            bool rez = Int32.TryParse(textBox1.Text, out myInt);
            if (rez)
            {
                if (myInt > maxSize)
                {
                    SetText("");
                }
            }
            else
            {
                // unable to convert string to integer.
                SetText("");
            }
            e.Handled = true;
        }

        delegate void SetTextCallback(String textToSet);
        private void SetText(String textToSet)
        {
            if (textBox1.InvokeRequired)
            {
                SetTextCallback d = new SetTextCallback(SetText);
                this.Invoke(d, new object[] { textToSet });
            }
            else
            {
                textBox1.Text = textToSet;
            }
        }