A condition preventing registration if the field does not contain at least a certain number of characters ?

Is this field a varchar (number + formating) or a integer (number only)?

If it is an integer field then you can specify the minimum and maximum value via Studio.

hello André, how can i specify the minimum value via Studio in my case ? 

hello André, how can i specify the minimum value via Studio in my case ? 

You can find it here. You need to make sure that the target field is an integer one, then both attributes Max Value and Min Value will be available.