That would set the maximum length. For minimum length you would be best off using javascript validation and then backing it up on the server side. There is no actual property within asp.net to set the Minimum Length, although mentioned above there is for 'MaxLength' (but take note that this will not work on Multi-Line textboxes (TextAreas), although the property still exists).
Use a regular expression to do it like..... <asp:RegularExpressionValidator ID="valPassword" runat="server" ControlToValidate="txtPassword" ErrorMessage="Minimum password length is 5" ValidationExpression=".{5}.*" /> Code (markup):