Sure, Rectangle<T>::getSize would return a Point<T>
It isn't quite right linguistically, because a point, a vector and a size are three different beasts that just happen to each have the same storage, i.e. 2 associated numbers.
But it works well in practice I think. It's a simple construct and it's obvious what's going on.
In my own work I use Vec2 Vec3 Vec4 as generic containers for 2 3 4 floats.
I vaguely remember having to use CGPoint and CGSize on iOS...