Well ot always!

I learnt to prohang a bit on an old Tutor 40 which has very little in terms of control surface size. I think the reason they are big is too give loads of throw (like 70 degrees) but with less drag than a smaller surfaces, and hence more response. Imagine a surface 1 wich wide, say an elevator. Than one 3 inches wide. Which one will give a smoother airflow? The 3 inch one.

Back to the topic title, I know its a 3d plane because it has got "3d" or "freestyle" in the name. Usualy the manufacturer describes it as a 3d plane anyway. A another good point, is weight, 5lb is usually the heaviest most 50 size 3d planes get though some weigh more. And also wingloading can tell you wether a 3d plane is a 3d plane or not. All this is linked to control surface size etc but more to wingpasn as well.

Im sure someone will build on what I've said here

Shahid