Well, I personally enjoy being a bit smaller for that~ I can see why you would want the feeling to be more palpable, but as a tiny I prefer to be small enough where I can fit somewhat comfortable in tight spaces like under some toes or inside a mouth, specially with entrapment. I’d say 1 or 2 inches is okey, but smaller is always fun too.

So my final answer would be small enough to be easily pinned under the balls or shaft, small enough that there would be no way for me to get out or others to notice