First thing to decide is the rev range youwant it to work over and how complicated a set up you want. The surge line to the choke limit is usually (very roughly) a factor of 2 in flow, newer turbos have a wider flow map, so a turbo that starts at 1000rpm will be just about done at 2000. One that starts at 3500 should see you right through to 7000, something in the range of a T3 or TD04 for an A series (again roughly, depending on the engine).

The advantage of a big turbo is that it works great up top generating the least heat and restriction the down side is it won't be very exciting down low. Porche chose the big turbo route and this is my preference but each to their own.

Have a look at Ray Hall Turbo charging, there's a calculator that will give you a very rough idea. A reasonably accurate and easy tored book is a turbocharging book by corky bell. I read a really complicated academic orientated book on turbocharging to written by a German Professor but unless you're into maths and heavy text it'll put you to sleep.