change the number of a character according to a choice field

Hello,
I have people from Belgium who register on my French site.
In France , the ZIP code consists of 5 digits and Belgium 4 digits.
I souihaiterai selected when what the country "Belgium" , the " postal code " field is reduced to 4 characters (default setting is 5 for France) . I try to do with the conditional CB and CB Query field but I turn around . I can not

Validation can't be conditioned. You'd need to have 2 separate fields then condition them to show based off the users selection or just allow up to 5 characters for the zip field.