Validate stringlength in dropdown

Hi

As the Subject says, i'd like to validate the stringlength in a dropdown.

As default the selected value in the dropdown is: '- Choose Customer - ' which is 19 characters.
All the other values that is possible to select is at least 40 charaters.

So i thought I could just use a RangeValidator and set MinimumValue to 20 and set the MaximumValue to a value the dropdown-text wouldn't exeed. And set the Type to a String.
Then I thought the validator would count the characters. But it didn't :-(

Any ideas on how to validate the dropdown?

Thanks, Thomas
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