picking turbo size is considered by most to be black magic. here is a chart that may help sort some of the magic out of selection. open the link and open the turbo matrix(it takes awhile).
http://www.treadstoneperformance.com/techinfo.phtml . select the displacement size on left of screen. next slide right and find the estimated hp you expect to make(be reasonable). keep in mind the turbo it shows you is for appr 10 psi boost as seen in upper left of screen. the corresponding turbo will have 2 parts. the first is the compressor side. there are many compressor sizes to4b, 60-1, 62-1 are 3 different sizes. they are sixed to the air volumn the engine can consume. next is the turbine wheel and housing. with turbonetics, they have wheels in diff sizes, such as stage V, O,P,Q trim sizes. the turbine housings also come in differing sizes such as .58, .63,.68,. 81 ect. with the wheel pick it with the chart as it is hard to change later. you can tune when the turbo boosts with the exh housing if needed. there are also a couple different style of housings to choose from for engine compartment room or placement. there is a tangential housing and a "on center" housing. if you where not confused before you otta be by now. check out the chart it will help. tom