can I override a jsr-303 validation annotation

You can ovveride an annotation configured validation via xml configuration:

http://docs.jboss.org/hibernate/validator/4.2/reference/en-US/html_single/#validator-xmlconfiguration

In your case, if you declare a different validation (or no validation) for the getList() methid in the validation.xml file, it will override the @Size annotation.

Just for the records, I have found a bypass for my problem:

public class TestSizeAnnotation
{
    public static void main(String[] args)
    {
        System.out.println("c " + 
                Validation.buildDefaultValidatorFactory().getValidator().validate(new C(), Default.class, SizeGroup2.class));
        System.out.println("p " + 
                Validation.buildDefaultValidatorFactory().getValidator().validate(new P(), Default.class, SizeGroup.class));
    }
    public static interface SizeGroup {}
    public static interface SizeGroup2 {}
    public static class P 
    {
        private List<String> lst = newArrayList("AA");
        @Size(max=0, message="P" , groups=SizeGroup.class)
        public List<String> getLst()
        {
            return lst;
        }
        public void setLst(List<String> lst)
        {
            this.lst = lst;
        }
    }
    public static class C extends P 
    {
        @Override
        @Size(max=5, message="C", groups=SizeGroup2.class)
        public List<String> getLst()
        {
            return super.getLst();
        }

    }
}

Overriding validation annotations is actually not supported for JSR-303. Instead annotations on overridden methods in the subclass will be applied cumulatively: From section 3.3 of the specification:

A constraint declaration can be placed on an interface. For a given class,
constraint declarations held on super- classes as well as interfaces are
evaluated by the Bean Validation provider. Rules are formally described in
Section 3.4.5.

The effect of constraint declarations is cumulative. Constraints declared
on a superclass getter will be validated along with any constraints defined
on an overridden version of the getter according to the Java Language
Specification visibility rules.