Try a regular expression:
public static Regex regex = new Regex(
"^([a-zA-Z]{1,6}$)|([a-zA-Z]{6}[\\w- ]*$)",
RegexOptions.IgnoreCase
| RegexOptions.Multiline
| RegexOptions.CultureInvariant
| RegexOptions.IgnorePatternWhitespace
| RegexOptions.Compiled
);
Match m = regex.Match(InputText);
if (m.Success)
{
...