The size of the turbine housing is the inlet cross-sectional Area divided by the Radius from the turbo centerline to the centroid of that area. This is marked as a number followed by A/R. This A/R number will vary depending on the size of the turbo and the intended use.
A lower A/R will forced exhaust gasses through a smaller opening around the turbine wheel. This will casue the wheel to spin faster at lower RPM's, but may physically choke the engine at higher RPM's. A higher A/R number will have a larger area for the gases to pass through the turbine wheel.
This will result in slower spool, but much higher flow and power at high RPM. At our altitude of 5000+ feet, we usually select the smallest A/R available for a particular turbo. This gives us the best spool times and low end torque.
Subaru Turbo Upgrades | This will entirely depend on the intended use of the turbo for which is better. External wastegate turbo tend to perform better than their internal gated counterparts, but take up considerably more room in the engine bay.
Other options are inlet and out size and flange type. For one size A/R, there might be 3 or 4 different flange options for a turbo.
Being very specific with the type of driving you are going to do pays off in the performance gains. There is a difference between high performance street application and a dedicated racing car turbo setup.