Bringing this thread back from the dead...
On the topic of turbo sizing, here's a few pictures I found that I think give a pretty plain explanation of all the measurements (trim, compressor size, a/r size).
A/R ratio:
This is a measure of volumetric efficiency. A water hose is the best example I've seen to explain this. Imagine you have a waterhose with no nozzle on it. You're watering your grandmothers's garden with it, and she has one of those stupid pin-wheels stuck in the ground. Your ADHD kicks in and you begin to run the water over the pinwheel. It spins like a geo metro with 22's. boring... So you grab the super-ultra-turbo nozzle from the garage and tighten it down. Aim and squeeze. The water jets out bashing the blades of the pin-wheel like a cinder block landing on a baby duckling. That pin-wheel now resembles a pro 5.0 half way down the track.
Putting that nozzle on, you're flowing less volume, but you've increased the pressure of the water exiting the hose.
So... larger a/r = more volume. More volume/larger a/r is great if you've got a stroked and bored 5.7L with more flow than the Mississippi river. A more minute sized motor like those in our Hondas, won't flow anywhere near this much, and the goal when choosing a turbo is to find one with maximum efficiency for your vehicle's flow capabilities. For example, a typical T3/TO4 57 trim turbo and a .48a/r housing might be most efficient between 3000-6000rpms, and capable of up to 300whp on an F23. The same turbo with a .63a/r might be between 3500~7300rpms, capable of 450hp, and a turbo with a .82 A/R housing might be good for 5000-9000rpms, and capable of 650hp. Though with the F23, anything over 6000rpms is nearly pointless unless you've had headwork done, and have an upgraded valve train as a minimum. Even then you might top out around 7200rpms, making a turbo larger than .63a/r very ineffective and inefficient for this particular motor. Still with me?
Let's move to Trim:
Trim is very simple. Trim is calculated by taking the square of the exducer's diameter, divided by the square of the inducer's diameter, and multiplied by 100.
Work in Progress... More to come...