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projection space object size

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what's the best way to calculate 3d objects size in projection/screen space? Is it fast enough to use this for culling? Is it used in modern game engines?

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How accurate do you need to be?
If you want perfect accuracy then you need to project every vertex to screenspace, finding the minima and maxima. I.e. not very efficient.

I typically use a bounding sphere representation, in which case i'm just projecting the left, top, right and bottom of the sphere.
I don't use this for culling however, I've needed screen space size approximation for impostoring tricks.
For culling of these, i just do sphere-vs-plane intersection tests (SIMD optimized)

Ultra modern game engines are looking to move culling either directly onto the GPU (Splinter Cell Conviction, iirc), or via a software hi-z approach (Frostbite 2)

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