It appears that no such constraint exists. furthermore, since I found out that I additionally need to validate the minimum length, I have chosen the solution of subclassing the Dijit.Form.ValidationTextBox.
This is the implementation I have chosen for the isValid method ,which reads from the constraints, so I can keep my generating XSL code consistent. )
isValid : function(){
var ancestorsValid = this.inherited(arguments);
if(ancestorsValid){
//Acquire only meaningful validation boundaries.
var minLength = this.constraints && this.constraints.minLength ? Number(this.constraints.minLength) : null;
var maxLength = this.constraints && this.constraints.maxLength ? Number(this.constraints.maxLength) : null;
//Validate min and max if present.
return ((minLength === null) || this.value.length >= minLength) &&
((maxLength === null) || this.value.length <= maxLength);
}
return false;
}