Count Cell Characters & If It Exists Already

I have a worksheet (SHEET1) where users would enter their ID Number and then they would hit a Submit button. The submit button would then add that ID Number into another worksheet (SHEET2) in a running list of ID Numbers. But before it is submitted, I would like to check for two things:

1. ID Number must be 9 characters long (numbers and/or letters)
2. ID Number must not already exist in SHEET2

If condition 1 is not met, then just have a message box asking user to check their entry and try again.

If condition 2 is not met, then just have a message box saying "thanks" but no addition will be made to SHEET2