Pop up warning message based on a certain value
 

I have data validation in cell A8 so that the user can only enter a string of
numbers six or seven digits long. Here is what I need to do:

If the user enters a string not equal to 6 or 7 digits, display the standard
"Stop" dialog box with a custom message so that they can fix it.

If the user enters a string equal to 7 digits, display the standard warning
message dialog box with a custom message and the standard options of Yes, No,
or Cancel.

If the user enters a string equal to 6 digits, do nothing.

Any and all help is greatly appreciated.

Thanks

Jeff