Small ones! Seriously, size matters in a turbo, but just asking for size doesn't mean anything. There are several different measurements. You have inducer and exducer diameter for both the turbine and the compressor wheel; the relationship is termed "trim". You also have the Area / Radius (A/R) which measures the inlet on the compressor and the outlet of the turbine. This is the cross sectional area of the inlet/outlet to the radius from the wheel centerline.
Putting that aside, deciding "big" vs "small" for any particular engine can be summed up by the RPM at which a turbo makes makes peak boost. Since the turbo to compressor feedback is exponential not linear, you'll normally go to peak boost with a 500-1000 of onset. On the ST, the dyno charts I have seen from both stock and different tuners show peak boost from 2500 to about 2800. That makes them "small" in my humble opinion.
